The outside
- Case: the metal housing that holds everything. Its width is the case diameter, but the lug-to-lug decides how it fits.
- Bezel: the ring around the crystal. Fixed and decorative, or rotating to time a dive, track a time zone or measure speed.
- Crystal: the clear cover over the dial, made of sapphire, mineral glass or acrylic.
- Dial: the face, carrying the hour markers, logo and any subdials or date window.
- Hour markers (indices): the marks for each hour, either applied as separate pieces or printed, often filled with lume.
- Hands: hour, minute and seconds, plus extra hands for a GMT or chronograph.
- Date window: an opening in the dial showing the date, sometimes magnified by a cyclops lens.
- Crown: the knob used to set the time and date and, on mechanical watches, to wind. Crown guards are the shoulders that protect it.
- Lugs: the arms that hold the strap. The gap between them is the lug width.
- Spring bars: spring-loaded pins between the lugs that hold the strap or bracelet; you compress them with a spring bar tool to change straps.
- Strap or bracelet: leather, rubber, fabric or a metal bracelet like the Oyster or Jubilee.
- Case back: the back cover, screwed or pressed on, sometimes with a sapphire window to show the movement.
The inside: the movement
The movement, or caliber, is the engine. A quartz movement is a battery, a quartz crystal, a circuit and a small motor. A mechanical movement, automatic or hand-wound, has these main parts:
- Mainspring: a coiled spring in a drum called the barrel. Winding tightens it; it stores the watch's energy for its power reserve.
- Gear train: the wheels that carry the mainspring's energy to the escapement and turn the hands at the right speeds.
- Escapement: the escape wheel and pallet fork, which release the energy in tiny, regular steps. It's what makes the ticking sound.
- Balance wheel and hairspring: the oscillator that swings back and forth several times a second and sets the pace, the heart of the watch's accuracy.
- Rotor: on an automatic, the weighted half-disc that spins with wrist movement and winds the mainspring.
- Jewels: synthetic rubies used as low-friction bearings at the points of most wear. A typical automatic has 21 to 27.
- Keyless works: the levers and gears that connect the crown to winding and hand-setting.
Anything beyond telling the time, like a date, chronograph or moon phase, is a complication. For every other term, see the watch glossary.