ATM / Bar Atmospheres and bar are units of pressure used to express water resistance. 1 ATM ≈ 10 meters of static water depth. A watch rated to 5 ATM / 50m is safe for swimming in calm water.
Automatic A watch movement that winds itself using the kinetic energy of the wearer's wrist, via a rotor. Also called 'self-winding.' No battery required.
Balance wheel The oscillating component at the heart of a mechanical movement. It swings back and forth at a precise rate (measured in beats per hour / bph), regulating the release of energy through the escapement.
Bezel The ring surrounding the watch crystal. Can be fixed (decorative) or rotating (functional — used for timing or tracking a second time zone).
BPH / Beat rate Beats per hour — the oscillation frequency of the balance wheel. Common rates: 21,600 bph (6 bps), 28,800 bph (8 bps). Higher rates allow smoother hand sweep and potentially higher accuracy.
Caliber The specific movement model inside a watch, identified by a number or code. E.g., Seiko NH36, ETA 2824-2, Orient F6724. Like an engine model number.
Case diameter The width of the watch case measured across the face, excluding the crown. The primary size measurement used to compare watches.
Caseback The back plate of the watch case. Can be screwed, snapped or bolted on. Exhibition (transparent) casebacks show the movement; solid casebacks may be engraved with specifications.
Chronograph A stopwatch function integrated into a watch, operated by pushers on the case side. Not the same as a chronometer.
Chronometer A watch certified to high accuracy standards by COSC (Contrôle Officiel Suisse des Chronomètres). Accuracy must be within -4/+6 seconds per day in multiple positions and temperatures. Tissot's Powermatic 80 is COSC-inspired but not COSC-certified.
Complication Any function beyond simple hours and minutes. Date, day, chronograph, GMT, moon phase and tourbillon are all complications.
COSC Contrôle Officiel Suisse des Chronomètres — Swiss organization that certifies movements to chronometer accuracy standards. A COSC-certified movement runs within -4/+6 sec/day across multiple test positions.
Crown The winding and setting knob on the side of the case. Crown positions: position 0 (pushed in, normal wear / winding on some models), position 1 (pulled to first click, date setting), position 2 (pulled fully, time setting).
Crystal The transparent cover over the dial. Can be sapphire (hardness 9, scratch-resistant), mineral (glass, more impact-resistant) or acrylic/Hesalite (soft but shatter-resistant).
Dial The 'face' of the watch — the decorated disc showing hour markers, hands, and any subdials or text. Sometimes called the 'face.'
Escapement The mechanism that controls the release of stored energy from the mainspring in regulated intervals, creating the characteristic 'tick' of a mechanical watch.
Exhibition caseback A transparent caseback (typically sapphire crystal) that allows viewing the movement. Often called an 'open caseback' or 'display caseback.'
Gear train The series of interconnected gears that transmits power from the mainspring barrel to the escapement, reducing speed and distributing force to drive the hands.
GMT hand A fourth hand completing one revolution per 24 hours, used to display a second time zone simultaneously with local time.
Hacking / Hack seconds A feature allowing the secondhand to stop when the crown is pulled out, enabling precise time-setting synchronized to a reference. Important for setting accurate time.
In-house movement A movement designed and manufactured by the same company that sells the watch. Orient and Seiko make their own calibers in-house. Not all brands do — many use ETA or other supplier movements.
Lug width The gap between the lugs where a strap or bracelet attaches, measured in mm. Determines strap compatibility. Common: 18mm, 20mm, 22mm.
Lugs The protruding arms on the case that hold the strap or bracelet, measured in the lug-to-lug distance (vertical case height) and lug width.
Lume / Luminous Photoluminescent material on hands and indices that glows in darkness after absorbing light. Modern lume (Super-LumiNova, LumiBrite) is non-radioactive and rechargeable.
Mainspring The coiled spring inside an automatic or manual movement that stores energy. Winding (manually or via the rotor) tensions the mainspring; the stored tension releases slowly through the gear train.
Manual-wind A movement without a rotor that must be wound by hand by rotating the crown. Also called 'hand-wind.' Common in vintage watches and some modern dress watches.
Power reserve How long a fully wound movement runs before stopping. Typically 38-80 hours for automatic watches. Tissot's Powermatic 80 leads at ~80 hours.
Pusher A button on the case side that operates a complication, typically a chronograph. Usually at 2 o'clock (start/stop) and 4 o'clock (reset).
Rotor The semicircular weighted disc in an automatic movement that swings with wrist movement, winding the mainspring. Visible through exhibition casebacks as it rotates.
Skeleton / Open-work A movement or dial with material cut away to reveal the inner workings. Full skeleton removes the dial entirely; open-heart cuts a window through the dial to reveal specific components.
Swiss Made A designation regulated by Swiss law requiring that the movement be Swiss-made, cased in Switzerland and inspected there. A quality indicator, not a performance guarantee.
TPD Turns per day — the rotation count setting on a watch winder. Different movements require different TPD settings; incorrect TPD can under-wind a watch or cause unnecessary movement wear.