Understanding watch terminology transforms confusing specifications into meaningful information. This glossary covers essential terms every watch enthusiast should know, from basic concepts to advanced complications.
A
Automatic Movement
A mechanical movement that winds itself using a rotor that spins with wrist motion. Also called "self-winding." No battery required—the watch runs on mechanical energy stored in the mainspring.
Amplitude
The arc of the balance wheel's swing, measured in degrees. Healthy automatic movements typically show 250-310 degrees. Low amplitude can indicate service needs.
Annual Calendar
A complication that automatically adjusts for months with 30 or 31 days, requiring manual correction only once per year (at the end of February).
B
Balance Wheel
The oscillating wheel that regulates timekeeping in mechanical watches. It swings back and forth at a consistent rate, typically 21,600 to 36,000 times per hour.
Bezel
The ring surrounding the watch crystal. May be fixed, rotating (for timing), or decorative. Dive watches feature unidirectional bezels that only rotate counterclockwise for safety.
Beat Rate (BPH)
Beats per hour—how many times the balance wheel oscillates hourly. Common rates: 21,600 (6 beats/second), 28,800 (8 beats/second), or 36,000 (10 beats/second). Higher rates generally mean smoother seconds hand motion.
C
Caliber
The specific movement design, usually identified by a number (e.g., Rolex Caliber 3235, ETA 2824). Indicates the movement's specifications and manufacturer.
Chronograph
A watch with stopwatch function. Pushers start, stop, and reset the timing function. Sub-dials display elapsed seconds, minutes, and sometimes hours.
Chronometer
A watch certified for accuracy by an official testing institute (usually COSC). Must meet strict precision standards over multiple days in various positions and temperatures.
COSC
Contrôle Officiel Suisse des Chronomètres—the Swiss official chronometer testing institute. Certifies movements meeting accuracy standards of -4/+6 seconds per day.
Complication
Any function beyond basic timekeeping. Date displays, chronographs, moon phases, and perpetual calendars are all complications.
Crown
The knob used to wind the watch and set the time. Screw-down crowns thread into the case for improved water resistance.
D
Date Window
An aperture displaying the date, typically at 3 o'clock. Some feature magnifying "cyclops" lenses for easier reading.
Dial
The watch face displaying time. Materials include metal, enamel, lacquer, mother-of-pearl, or stone. The dial holds indices (markers) and supports the hands.
Deployment Clasp (Deployant)
A folding clasp that opens the bracelet without fully detaching it. Provides security and reduces wear on leather straps.
E
Escapement
The mechanism that controls energy release from the mainspring to the balance wheel, creating the characteristic "tick" sound. Types include lever, co-axial, and deadbeat escapements.
ETA
The largest Swiss movement manufacturer, supplying movements to many brands. Common calibers include ETA 2824 (automatic) and ETA 7750 (chronograph).
F
Flyback
A chronograph feature allowing instant reset and restart with a single button press, without stopping first. Useful for timing consecutive events.
G
GMT
A complication displaying a second time zone using an additional hand and 24-hour bezel. Originally designed for pilots crossing time zones.
Guilloche
A decorative pattern of precise, interlocking lines engraved on dials. Traditional guilloche is engine-turned by machine; modern versions may be stamped.
H
Hacking
A feature that stops the seconds hand when the crown is pulled out for time-setting. Allows precise synchronization to a reference time.
Hand-winding
A mechanical movement wound manually by turning the crown. Also called "manual wind." Requires daily winding to maintain power.
Helium Escape Valve
A valve allowing helium to escape from the case during decompression after saturation diving. Found on professional dive watches rated beyond 300m.
I
In-House Movement
A movement designed and manufactured by the watch brand itself, rather than sourced from external suppliers. Indicates higher investment in manufacturing capability.
Index
Hour markers on the dial—may be applied (raised), printed, or painted. Types include stick indices, Arabic numerals, and Roman numerals.
J
Jewels
Synthetic rubies used as bearings in movements to reduce friction. More jewels isn't necessarily better—17-25 jewels is typical for quality automatics. Unnecessary jewels are sometimes added for marketing.
L
Lug-to-Lug
The measurement from the top of one lug to the bottom of the opposite lug. Determines how a watch fits on the wrist—sometimes more important than case diameter.
Lume (Luminous Material)
Phosphorescent material applied to hands and indices for visibility in darkness. Modern watches use Super-LumiNova; vintage watches may contain radioactive tritium or radium.
M
Mainspring
The coiled spring storing energy in mechanical watches. When wound (manually or automatically), it powers the movement as it slowly unwinds.
Manufacture
A watch company that produces its own movements in-house, rather than sourcing from suppliers. True manufacture status indicates significant vertical integration.
Moon Phase
A complication displaying the current lunar phase through an aperture showing a moon disc rotating beneath. Astronomical moon phases cycle every 29.5 days.
P
Perpetual Calendar
A complication that automatically accounts for different month lengths and leap years, requiring no manual adjustment until 2100 (when the leap year exception occurs).
Power Reserve
How long a mechanical watch runs when fully wound before stopping. Typically 38-72 hours for modern automatics. Some watches display remaining power reserve on the dial.
Pusher
Buttons on the case used to control functions—typically chronograph start/stop and reset. Distinguished from the crown, which sets time.
Q
Quartz Movement
A battery-powered movement using a vibrating quartz crystal for timekeeping. More accurate than mechanical movements (+/- 15 seconds per month typical) but lacks the craftsmanship appeal of mechanical watches.
R
Rotor
The semicircular weight in automatic movements that spins with wrist motion to wind the mainspring. Visible through exhibition casebacks.
S
Sapphire Crystal
Synthetic sapphire used for watch crystals—extremely scratch-resistant (9 on Mohs scale, after diamond). May be flat, domed, or box-shaped. Can shatter on impact unlike acrylic.
Skeleton
A watch with material removed from the movement and/or dial to reveal the mechanism beneath. Shows the working parts but may reduce durability.
T
Tourbillon
A rotating cage containing the escapement, designed to counter gravity's effects on accuracy. Originally invented for pocket watches; now primarily a demonstration of craftsmanship rather than practical necessity in wristwatches.
W
Water Resistance
Rated in meters, atmospheres (ATM), or bars. 30m = splash resistant; 100m = swimming; 200m+ = diving. Ratings assume static pressure—actual use involves additional forces.
Winding Stem
The shaft connecting the crown to the movement, transmitting winding and time-setting inputs.