Watch Terms Glossary

A
Anti-reflective coating

Anti-reflective coatings enhance a watch crystal's light transmission and clarity. This results in fewer reflections off the crystal, thus improving readability.

Antimagnetic watch

An antimagnetic watch will keep functioning unimpeded when exposed to magnetic fields of a particular strength. To qualify, it must maintain a certain level of accuracy according to ISO 764.

Automatic

Automatic refers to the automatic winding of a watch caliber. The motion of the wearer's wrist and arm causes a weight (the rotor) to oscillate, thus winding the mainspring. Mainsprings have a slipping clutch device to protect them against damage caused by overwinding. The central rotor mechanism is very common.

B
Bakelite

Bakelite is the trade name for a fully synthetic plastic created by Belgian-American chemist Leo Baekeland at the start of the 20th century. Objects such as steering wheels, radios, phones, and the handles of pots and pans have been made from this heat-resistant material.

Balance wheel

The balance wheel regulates the beat of a mechanical watch through its constant vibrations. It consists of a circular balance rim. Many definitions include the hairspring as part of the balance wheel as well. The balance wheel takes over the job of the seconds pendulum found in grandfather clocks and wall clocks; however, it vibrates at a much faster rate. Today, normal speeds are either 21,600 or 28,800 vibrations per hour (vph), while a seconds pendulum only ticks at 3,600 vph. How precisely a watch runs depends on the number and regularity of the vibrations. The escapement gradually provides the balance wheel with energy from the mainspring, causing it to oscillate back and forth.

Bar

A bar is a metric unit of pressure that indicates how much weight is being applied to a surface. Other units of measurement for pressure are the standard atmosphere (atm) and the pascal (Pa).

1 bar = 100 kPa = 0.1 MPa

1 bar is about equal to the atmospheric air pressure on the Earth's surface at an altitude of 111 m (364 ft) at 15°C (59°F). This is equivalent to the pressure experienced underwater at a depth of 10 m (33 ft). At sea level, the average pressure is 1.013 bar.

Barrel

The barrel contains the mainspring. The mainspring stores the energy created by winding the watch.

Baumuster (Type)

Baumuster (or English "type") refers to a standard construction and dial design for pilot's watches created by the German Air Force during the Second World War. There are two types: Baumuster A and Baumuster B.

Bezel

The bezel is a ring that surrounds the watch crystal. It can be rotatable or stationary. Diving watches feature unidirectional rotatable bezels with minute markers for keeping track of dive times. Chronographs often have fixed bezels with a tachymeter scale for calculating average speed. Bezels are typically made of metal or ceramic.

Breguet overcoil

A Breguet overcoil (or Breguet balance spring) is a balance spring with a slightly raised final coil that has a reduced curvature. Abraham-Louis Breguet invented this component in 1795. Its concentric form allows the spring to "breathe" better and keeps the watch running more precisely.

"
"Swiss-made" law

The Swiss Federal Act on the Protection of Trade Marks and Indications of Source defines what constitutes a "Swiss-made" product. As of 2017, for a watch to earn the designation of "Swiss made," 100% of the movement's assembly must take place in Switzerland, while only 60% of the manufacturing must occur within "the Swiss region." The law does not consider straps and bracelets part of the watch.

C
Calendar watch

Any watch with one or more calendar displays. Varieties include:

Year display

The year display shows the current year on the dial.

Date display

Most date displays take the form of either an additional hand (pointer date) or number discs below a cutout in the dial. The hand or number discs complete one full rotation every 31 days. These displays require manual correction at the end of months with fewer than 31 days.

Day-date display

The day-date display is an expanded date display that includes the day of the week. Most day displays appear either at 12 o'clock or next to the date at 3. The Rolex Day-Date is one of the most famous watches with this display type.

Perpetual calendar

A perpetual calendar is a watch complication that displays the correct date according to the Gregorian calendar. A perpetual calendar can account for varying month lengths, including leap years. Current perpetual calendars will be accurate until the year 2100, at which point they will require a single manual correction.

Triple calendar

A triple calendar function has displays for the day, date, month, and year. If it also features a moon phase indicator, it is called a complete calendar.

Annual calendar

An annual calendar is a complication whose date only has to be corrected manually once per year at the end of February.

Caliber

Caliber is another term for a watch movement that's often used in combination with a numerical movement name, such as "ETA caliber 2824-2." Sometimes spelled "calibre."

Central seconds

A watch has a central seconds when its second hand is on the same central axis as the minute and hour hands. It is an alternative to the small seconds, which displays the seconds in a subdial, usually at 6 o'clock. Small seconds are often found on chronographs, which use the central second hand as the chronograph second hand.

Cerachrom

Cerachrom is Rolex's proprietary ceramic mixture. This state-of-the-art material is especially hard and scratch-resistant. Rolex watches with Cerachrom bezels appear in the GMT-Master II and Submariner collections, among others.

Chamfering (Anglage)

Chamfering, also called beveling or anglage, is a complicated method for refining watch movements. It involves shaving the edges to a 45° angle and giving them a polished finish. The width of the edges remains the same.

Chime

A chiming mechanism is a separate mechanism in a mechanical watch. A hammer hits a resonating body, such as a gong or chime, to relay the time acoustically.

Chronograph

Chronographs have a stopwatch function for timing things like sporting events. Most chronographs feature a pusher on either side of the crown, as well as additional subdials for measuring elapsed time. A central chronograph seconds hand tracks the elapsed seconds. Famous chronographs include the TAG Heuer Autavia and Omega Speedmaster.

Chronometer

A chronometer is an especially precise caliber that has earned a certification of precision from an official body. The Official Swiss Chronometer Testing Institute (French: Contrôle officiel suisse des chronomètres, COSC) is the most famous chronometer testing body. There's also a chronometer testing facility (Chronometerprüfstelle) at the Glashütte Observatory in Glashütte, Germany.

Clasp

The clasp holds the bracelet or strap together, thus securing the watch to the wrist. It is usually made of stainless steel or other metals. There are different clasp types, which can be classified as follows:

Pin buckle (Tang buckle)

A pin buckle is a type of clasp commonly found on leather watch straps. The longer end of the strap has holes punched in it. The shorter end has the actual pin, as well as a spring bar and a metal holder in the shape of a U, similar to a belt buckle. It functions in a similar fashion as well: The pin is inserted into one of the holes to achieve the desired length. The metal holder keeps the pin from coming out of the hole.

Double deployant clasp (Butterfly clasp)

Double deployant clasps are folding clasps with two hinged sections, providing a wide opening. When closed, the clasp is hidden from view, giving the illusion of one continuous metal bracelet. However, on leather straps, the buckle is still visible. Push pieces on either side of the buckle release the clasp.

Folding clasp (Deployant clasp)

Most stainless steel bracelets feature a classic folding clasp with brackets. One famous example is Rolex's Oysterlock clasp.

Co-axial escapement

English watchmaker George Daniels invented the co-axial escapement as an alternative to the Swiss lever escapement in the 1970s. Its name comes from its two escape wheels mounted on a shaft, one above the other. This design results in a significant reduction in friction between the two wheels. Therefore, the escapement system requires less lubrication and runs longer before requiring maintenance. Omega further developed the co-axial escapement into a series of watches in the late 1990s. The majority of current mechanical Omega watches have calibers with this escapement system.

Complication

A complication is an additional watch function. Moon phase indicators, alarms, stopwatches, and perpetual calendars are all common complications. They pose a challenge for watchmakers, especially when there are multiple complications in one watch movement.

Compressor (Super compressor)

A compressor (or super compressor) is a trademarked case construction with a unique sealing method. Watch case manufacturer Ervin Piquerez S.A. (EPSA) developed and patented the design in the mid-1950s. The unique design uses the pressure experienced underwater to compress the case against an o-ring gasket, forming a seal. The greater the depth, the higher the pressure, and the tighter the seal. The Longines Legend Diver is an example of a popular compressor watch.

D
Display case back

Display case backs appear on high-end wristwatches and are typically made of sapphire crystal or mineral glass. They provide a view of the movement at work.

Diving watch

A diving watch (also known as a dive watch, diver's watch) is suitable for use while diving recreationally or professionally. The most common standards used for diving watches are ISO 6425 and DIN 8306. The minimum required water resistance is 100 m (10 bar, 328 ft). High-quality diving watches are usually water-resistant to at least 200 m (20 bar, 656 ft), have luminous hands and indices, and feature a bezel with minute markers. The bezel only rotates in one direction to avoid the wearer accidentally lengthening their dive time. Some diving watches can withstand depths of 1,000 m (3,280 ft) or greater; these usually have a helium escape valve as well.

Double barrel

A double barrel is a movement with two mainsprings and barrels. This extends the watch's power reserve.

Double chronograph

See "Split-seconds chronograph"

Drift angle calculator

The term "drift angle" refers to a deviation from an intended path and is the result of crosswinds. The wearer can use a drift angle calculator to determine their drift and adjust their path accordingly. Drift angle calculators are most often used in aviation and seafaring. One of the most famous watches with this function is the Hamilton Khaki X-Wind.

Dubois Dépraz

Dubois Dépraz is a specialist manufacturer of watch complications. For example, Omega uses the Dubois Dépraz chronograph module 2020 in the caliber 3220, which powers the Speedmaster Reduced Automatic.

E
ETA SA Manufacture Horlogère Suisse

The ETA SA Manufacture Horlogère Suisse (ETA SA Swiss Watch Manufacturer) is a Swiss watch movement manufacturer belonging to the Swatch Group.

Escape wheel

The escape wheel is a part of a watch movement with a lever escapement. It sits between the gear train and the balance wheel, connected to the balance wheel via the pallet fork. Escape wheels have asymmetrical teeth.

Escapement

The escapement ensures a steady, controlled release of energy from the mainspring. This mechanism locks and unlocks the gear train in even steps while also transferring new energy to the oscillation system.

Today's wristwatches predominantly use the Swiss lever escapement, which consists of a pallet fork and an escape wheel. The escape wheel connects directly to the fourth wheel, which attaches to the second hand via an axle. The balance wheel swings back and forth, setting the pallet fork in motion. The fork then catches and releases the escape wheel with a pallet, moving it forward one tooth at a time. At a balance frequency of 28,800 vph (4 Hz), the second hand takes eight steps forward per second.

Everose gold

Everose gold is Rolex's 18-karat rose gold alloy. Its platinum content helps keep the alloy brilliant longer than typical rose gold, and the pink color comes from copper.

F
Finishing

Finishing (French: finissage) refers to the refinement of watch movements. Common finishes include Geneva stripes, perlage, and sunbursts. Tempered blue screws and chamfering are also forms of finishing.

Flyback chronograph

The flyback chronograph is a special kind of mechanism by which the wearer can push a button once to set the stopwatch back to zero and start it again while it is running. This technology is a product of military aviation, where pilots must execute multiple maneuvers in quick succession at exactly the right second. By contrast, standard chronographs require three pushes to achieve the same thing: one to stop the chronograph, another to reset it, and a final push to restart it – a process that would take far too much time in the heat of battle.

G
GMT

GMT is the abbreviation for Greenwich Mean Time. It refers to the astronomically defined official time of Greenwich, a London borough located on the Prime Meridian. It previously served as the primary time standard until it was replaced by Universal Coordinated Time (UTC) in 1972. While often used synonymously, there are slight differences between GMT and UTC.

A GMT watch is capable of displaying the time in two time zones at once. Famous GMT watches include the Rolex GMT-Master and Glycine Airman.

Gauss

Gauss is a unit of measurement for magnetic induction. It's named after German mathematician and physicist Carl Friedrich Gauß (or Gauss). Watch manufacturers use Gauss to indicate the strength of magnetic field a watch can withstand without suffering side effects. Famous anti-magnetic watches include the IWC Ingenieur, Rolex Milgauss, and Omega Seamaster Aqua Terra 15,000 Gauss.

Geneva Seal

The Geneva Seal is a seal representing the origin and quality of a caliber. Traditionally, the seal was stamped into the metal of the movement. However, a new method of nanostructural marking changes the metal on a microscopic level. Therefore, even the smallest movement components can carry the Geneva Seal. To qualify for a Geneva Seal, the assembly, adjustment, and encasing of a mechanical caliber must all occur in the Canton of Geneva. There are 12 additional criteria relating to finishing, quality, and materials used, which the caliber must also fulfill. Eight members from the Office for the Optional Control of Watches of Geneva (French: Bureau Officiel de l'Etat pour le contrôle facultatif des montres de Genève) are in charge of granting seal approval to watches. Cartier, Vacheron Constantin, Roger Dubuis, and Chopard are some of the most famous manufacturers whose movements have Geneva Seals.

Geneva stripes

Geneva stripes are straight, broad stripes that adorn movements and sometimes other watch components as decoration. Also known as Côtes de Gèneve or filets.

Guilloché dial

Guilloché is a style of engraving done by machine or hand that is often used to decorate watch dials. The result is a beautiful pattern of interwoven and overlapping lines. Famous examples of guilloché dials appear on the Audemars Piguet Royal Oak and Maurice Lacroix Aikon.

H
Hairspring

The hairspring (also known as the balance spring) sits within the balance wheel and belongs to a mechanical watch's oscillation system. It expands and contracts multiple times per second and determines the watch's frequency. The hairspring is thinner than a human hair and weighs only two milligrams. It's made of a special material, such as the alloy Nivarox or silicon, an anti-magnetic metalloid.

Hand-guillochéd dial

Hand-guillochéd dials feature a hand-engraved guilloché finish. Since it's done manually, there are tiny irregularities in the pattern's lines.

Helium escape valve

A helium escape valve protects a diving watch against potential damage from excess pressure. Professional divers breathe in a special gas mixture that includes helium when in a decompression chamber. If they are wearing a watch, these tiny helium atoms can find their way into the case. The pressure of these atoms can then cause the watch crystal to pop out when divers return to normal pressure levels. The helium escape valve allows the gas to exit the case, thus equalizing the pressure and preventing damage. It functions either automatically or manually. Popular watches with helium escape valves include the Rolex Sea-Dweller and Omega Seamaster Planet Ocean.

I
Index (indices)

Indices (singular: index) are used to mark periods of time on a dial. This usually includes 12 hour indices and a further 60 indices for the minutes. Indices can be a geometric shape or a numeral and are either applied or stamped on the dial.

L
Lug

The lug is a component that connects the metal bracelet to the watch using spring bars. Lugs are generally made to fit a specific watch and cannot be changed.

Lug width

The lug width is the gap between a watch's lugs, which determines the width of the bracelet or strap.

Luminous material (Lume)

Radium

The radium isotope Ra-226 is a radioactive element that glows brightly in the dark and was used to illuminate watch dials until the 1960s.

SuperLuminova

SuperLuminova is a brand name for a photoluminescent pigment. Many manufacturers use this substance to coat watch hands and indices. Light activates the material, which glows green in the dark. However, its brightness fades over the course of several hours. Unlike tritium and radium, SuperLuminova is non-radioactive and nontoxic. Furthermore, SuperLuminova is chemically stable, meaning it can function properly for many years.

Tritium

Tritium is a slightly radioactive hydrogen isotope. Mixed with pigments containing zinc sulfide, it replaced radium as the predominant luminous material before falling out of use in the mid-1990s.

Luminous numerals

Luminous numerals are coated with a material that glows in the dark. In the past, the glow came from radioactive substances like radium and tritium. Today, the main material used is SuperLuminova. This non-radioactive material is made up of inorganic, phosphorescent pigments called lume. Once an artificial or natural light source has sufficiently activated the pigments, they begin to glow.

M
Mainspring

The mainspring stores the energy that powers a mechanical watch. It's located in the barrel and is wound either manually or, in the case of automatic watches, via a rotor. A movement's escapement prevents the mainspring from transferring all of its energy to the gear trains and balance wheel at once and ensures a controlled release over several days.

Manual

Manual winding is a method of providing a watch movement with energy. The user winds the mainspring by hand via the crown (hand-wound). The spring then transfers the supplied energy to the gear train.

Master Chronometer

The term "Master Chronometer" refers to a watch that has passed a special test conducted by the Swiss Federal Institute of Metrology (METAS). Omega has their Master Chronometers certified this way. These Omega calibers also feature a co-axial escapement, COSC certification, and magnetic resistance to at least 15,000 gauss.

Minute repeater

The minute repeater is a complication that relays the time acoustically via a miniature set of chimes. This incredibly intricate mechanism is one of the rarest watch complications.

Moon phase

The moon phase indicator is a watch complication that shows the current phase of the Moon as seen from Earth, from the new moon to the full moon and back. One lunar month lasts 29 days, 12 hours, 44 minutes, and 2.9 seconds. Most moon phase indicators use a window with two small humps to accurately display the Moon as it waxes and wanes.

Moonwatch

Moonwatch is another name for the Omega Speedmaster Professional. Back in 1969, astronaut Buzz Aldrin was wearing his NASA-issued Speedmaster as he became the second man to step foot on the Moon. The current version of the classic Moonwatch bears the reference number 311.30.42.30.01.005.

N
NATO strap

NATO straps are made of two pieces of material: One wraps around the wrist, while the other secures the case in place. Originally developed for the military, modern NATO straps are available in nylon, canvas, or leather. NATO straps attach to the lugs via a spring bar.

P
Pallet fork (Lever)

The pallet fork is a component of the escapement shaped like the bottom of a ship's anchor. It connects the escape wheel to the balance staff. The pallet fork locks the escape wheel and gear train in place, releasing it one tooth at a time with every oscillation of the balance wheel.

Parachrom

The Parachrom hairspring (see also: Breguet overcoil) is a patented Rolex innovation. It is made of a niobium-zirconium alloy. Oxidation turns the metal's outermost layer blue. The term "Parachrom" is a portmanteau of the words "paramagnetic" and "chrome."

Pilot's watch

The pilot's watch first appeared in the early 20th century and is one of today's most popular watch types. Most pilot's watches are modeled after dashboard instruments from early aircraft. Famous pilot's watch manufacturers include IWC, Hanhart, Tutima, and Sinn.

Power reserve

The power reserve is the amount of time it takes the movement to run out of energy after it has been fully wound, without being rewound by hand or body movements.

Power reserve indicator

The power reserve indicator shows how much time is left until a mechanical movement will run out of energy. It lets the wearer know if and when they should wind the watch. Many watches can be wound via the crown.

Q
Quartz watch

Quartz watches are powered by a quartz crystal. A battery sends an electric current through the crystal, which causes it to vibrate at a constant rate of 32,768 Hz. These vibrations are then converted into one electronic pulse per second. This drives a stepping motor to turn the gear wheels that control the watch's hands. Quartz watches from Asia took the world by storm in the 1970s thanks to massive supply and attractive prices. They toppled the traditional watch industry during the so-called Quartz Crisis. Some quartz watches feature solar receptors to charge their batteries.

Quickset date feature

The quickset date feature allows wearers to easily set the date with the crown pulled out. Movements without this feature require the hour hand to make two full rotations before setting the date.

R
Reference number

The reference number is equivalent to a model number in the watch world. It serves as the watch's unique identification. The reference number is helpful when searching for a specific watch.

Regulator

The regulator is used to fine-tune the balance spring for maximum precision. This should take place in different positions and at various temperatures. For example, official chronometer testing bodies require movements to be adjusted in at least five positions at three temperatures.

Rehaut ("Edge of the dial")

The rehaut is the chamfered edge of the dial that touches the watch glass. It's often used for scales and engravings.

Repeater

The repeater is a complication for telling the time acoustically. Mechanical calibers use a chiming mechanism triggered by an additional lever or push-piece on the side of the case. There are five types of repeaters: hour, quarter, half-quarter (one-eighth), five-minute, and minute. Timepieces with a repeater are extremely valuable due to their complexity.

Rolesor

Rolex uses the term "Rolesor" for watches that combine stainless steel and gold. A more common term for watches made of two metals is "two-tone."

Rotatable bezel

A bezel is a ring surrounding the dial and crystal. Rotatable bezel varieties often appear on diving or pilot's watches, for example.

Diving watches have rotatable bezels that can only rotate counterclockwise. This prevents the wearer from accidentally turning the bezel and lengthening their dive time. Before going underwater, the diver synchronizes the zero marker to the minute hand. The bezel's 60-minute scale then enables them to track how much time has passed. Pilot's watches feature bidirectional rotatable bezels.

Rotor

The rotor is a flexibly mounted, semicircular metal component that belongs to the winding mechanism of an automatic watch. When the watch moves, the rotor swings, thus winding the mainspring.

S
Screw-down crown

A screw-down crown screws tightly into the watch case and offers improved water resistance compared to crowns that are only pushed into the case. The Rolex Oyster, introduced in 1926, was the first wristwatch with a screw-down crown.

Screw-down push-pieces

Like a screw-down crown, screw-down push-pieces screw tightly into the watch case. The mechanism increases the case's water resistance. Screw-down push-pieces are especially common on watches that are water-resistant to extreme depths, such as the Sinn 203.

Sedna gold

Sedna gold is a reddish, 18-karat alloy made by Omega. It is composed of gold, copper, and palladium.

Sellita

Sellita Watch & Co. SA is a Swiss manufacturer of mechanical watch movements. Sellita calibers are an alternative to ETA movements and appear in many Swiss watches. Many Sellita movements are based on their ETA counterparts. The SW 200 is a famous three-hand Sellita caliber.

Shaped watch

A shaped watch is any watch that isn't the typical circular shape. Common varieties include cushion-shaped, rectangular, and square cases.

Shock protection

Shock protection is a system that protects fragile watch components against damage caused by hard impacts. The balance wheel's pivots are especially delicate and susceptible to damage. A tiny metal spiral absorbs the shocks. A watch is considered shock-resistant when it can survive a fall from a height of 1 meter onto a horizontal hardwood surface and suffer no damage. The most common shock protection system is Incabloc, although some manufacturers use proprietary systems.

Skeleton watch

A skeleton watch is a watch that has numerous cutouts in its movement, enabling the wearer to see through the timepiece. Skeleton watches and clocks are usually fine pieces of art and, thus, very complicated to create.

Small seconds (Subsidiary seconds)

The small seconds subdial displays the current seconds and is a common feature on pocket watches, manual wristwatches, and chronographs. The small seconds is an alternative to the central seconds, where the second hand shares its axis with the minute and hour hands in the middle of the dial. One of the most famous watch movements with a small seconds is the Unitas 6497.

Smartwatch

A smartwatch is a wearable computer shaped like a wristwatch. Modern smartwatches feature touchscreens for adjusting settings, as well as apps for monitoring things like physical activity or the wearer's heartbeat.

Split-seconds chronograph

A split-seconds (or double) chronograph can time intervals. To do so, it has two chronograph second hands and three push-pieces. One push-piece starts both second hands. A second push-piece stops one of the second hands and allows you to read how much time has passed while the other second hand keeps running. The third push-piece starts the stopped second hand again. Also known as a rattrapante chronograph. Not to be confused with the flyback chronograph.

Spring Drive

The Spring Drive is a piece of watch technology developed by Seiko. It replaces the classic escapement in favor of an electronic regulation system that provides energy to the mainspring.

Spring bar

A spring bar is a spring-loaded tube that attaches the bracelet or strap to the case.

Stainless steel

Stainless steel refers to alloy or non-alloy steel with a particular purity level. When it comes to watches, it's important to use rustproof stainless steel to protect against corrosion.

The most common alloy in the watch industry is 316L stainless steel. Rolex utilizes 904L stainless steel. These rustproof alloys contain chromium and nickel and are especially resistant to acids and moisture.

Stop-seconds mechanism

A stop-seconds mechanism enables the wearer to set the watch to the exact second. Pulling out the crown triggers the mechanism, which causes the second hand to stop. Once set to the correct time, the crown is pushed back into its original position, and the second hand starts moving again.

Subdials

Some dials may feature one or more subdials (registers) that display additional information. For example, small seconds subdials often appear on watches with two central hands. Most chronographs have two or three subdials. Some manufacturers may refer to these models as bi-compax (two subdials) or tri-compax (three subdials). Common layouts include 3 and 9 o'clock for two-subdial watches and 3, 6, and 9 o'clock for three-subdial watches. Famous examples include the Eberhard Contograf and the Rolex Daytona with two and three subdials, respectively.

Swan-neck fine adjustment

This type of fine adjustment uses a spring shaped like a swan's neck to regulate the watch. It allows for especially precise regulation, hence the term "fine adjustment." Watch manufacturer A. Lange & Söhne famously uses this mechanism.

T
Tachymeter scale

Tachymeter scales enable the calculation of units per hour. These scales usually appear on either the bezel or the edge of the dial. They are mostly used for calculating speed (km/h or mph). For example, if you drive a kilometer while timing yourself with your chronograph and it takes you 28 seconds, the tachymeter scale will show a speed of 130 km/h. Famous watches with a tachymeter scale include the Omega Speedmaster Professional and the Rolex Daytona.

Telemeter scale

Telemeter scales sit on the edge of a chronograph's dial and are used to calculate distance. For example, to determine how far away a storm is, the wearer would start the stopwatch when they see lightning and then stop it when they hear thunder. The large stopped chronograph second hand will be pointing to the correct distance on the scale. The scale is also useful with artillery; you can use it to determine how far away enemy troops are based on the time between the muzzle flash and the bang.

Tempering

Tempering refers to the process of slowly heating steel components until they reach the right temperature. This results in a thin colored coating on the metal's surface. Many manufacturers choose a shade of cornflower blue, which requires the metal to be heated to 300°C (572°F). Watchmakers use this process to refine hands, screws, and other components. Tempered components are particularly common on watches produced in Glashütte, Germany.

Tourbillon

A tourbillon is a round cage that rotates around itself once per minute. The most important parts of a mechanical watch sit within this cage: the oscillation and escapement systems. Gravity can influence these systems and cause small deviations when a watch remains in a vertical position, so the tourbillon's rotation compensates for these deviations. Abraham-Louis Breguet invented the tourbillon for pocket watches in 1795. Today, tourbillons are most common in high-end luxury watches since their production demands a high degree of skilled craftsmanship.

Tropical dial

Due to exposure to outside influences, a dial's appearance can change over time. The result is a so-called "tropical dial." For example, UV rays can slowly turn a black dial brown. Technically a defect, models with tropical dials, such as vintage Rolex Sea-Dwellers and Submariners, are especially popular among collectors.

Two-tone

The term "two-tone" refers to watches that combine two types of metal, most commonly stainless steel and gold. Popular two-tone watches include the Rolex Submariner and the Longines Hydroconquest.

V
Vertical integration

In watchmaking, vertical integration refers to a company's ability to produce their components in house rather than sourcing them from third parties. Famous vertically integrated companies include A. Lange & Söhne, Rolex, and Seiko.

W
Watch crystal

Acrylic glass

Acrylic glass is a synthetically produced, transparent material that is easy to manipulate in certain temperature ranges. Acrylic glass is very weatherproof, as well as resistant to breakage and corrosion. Small scratches can be easily polished away.

Hardlex crystal

Hardlex crystal is a type of mineral glass predominately found on Seiko watches. Thanks to a special production process, it is harder and more scratch-resistant than conventional mineral glass. In terms of durability, it falls between mineral glass and sapphire crystal.

Hesalite

Hesalite is Omega's name for plexiglass. It's inexpensive to produce and replace and doesn't splinter.

Mineral glass

Mineral glass is the standard crystal material used in the lower and mid-price ranges. It's comparable to window glass and harder than acrylic glass but softer and less scratch-resistant than sapphire crystal. Mineral glass can be hardened to improve its qualities. Also known as mineral crystal.

Plexiglass

Identical to Hesalite and acrylic glass

Sapphire crystal

Sapphire crystal is a synthetic product. It's significantly harder and more scratch-resistant than mineral or acrylic glass and is, therefore, predominantly used in luxury watches.

Water resistance

When indicating water resistance, manufacturers often provide measurements for both pressure (bar) and maximum depth (meters and/or feet). However, the depth value can be misleading: Watches with a resistance of 30 m (3 bar, 98 ft) aren't even rated for swimming and can only withstand splashes of water. Most diving watches are water-resistant to at least 200 m (20 bar, 656 ft). Water-resistance depends on more than just water pressure; temperature fluctuations can also be a factor. A watch's water resistance should be checked regularly, as gaskets wear out over time. Any moisture that seeps into the case usually first appears as condensation on the watch crystal and can destroy the movement.

Winding mechanism

The winding mechanism winds the mainspring. Pocket watches used to require a key for winding the mainspring (key-wind). Later, this was replaced with the crown (stem-wind). In an automatic timepiece, an oscillating weight known as the rotor performs this function.

World time watch

World time watches have a city disc that can display the time in at least the 24 whole-hour time zones. A crown or pusher is used to select the time zone's representative city, while a hand displays the time in that time zone.