Case and size

Case diameter
The width of the watch case, usually measured across the case without the crown. The number brands quote first, and a weak guide to how a watch wears. Why diameter misleads
Lug-to-lug
The distance from the tip of the top lugs to the tip of the bottom lugs. The watch's real length across your wrist, and the measurement that decides fit. Lug-to-lug explained
Lugs
The arms that project from the case at 12 and 6 o'clock and hold the strap or bracelet.
Lug width
The gap between a pair of lugs, where the strap attaches. It sets which straps fit: usually 18, 20 or 22 mm. Watches by lug width
Case thickness
The height of the watch from the back to the top of the crystal. Thinner watches slide under a cuff and sit closer to the wrist.
Bezel
The ring around the crystal. It may be fixed and decorative, or rotate to time dives, track a second time zone or count down.
Crown
The knob on the side of the case used to set the time and, on manual and many automatic watches, to wind the movement.
Screw-down crown
A crown that threads into the case to seal it, common on dive watches. Unscrew it before setting the time.
Case back
The back of the case. A display case back has a sapphire window that shows the movement.
Crown guards
Shoulders on the case that protect the crown from knocks.

Dial and crystal

Crystal
The clear cover over the dial: sapphire, mineral glass or acrylic.
Sapphire crystal
Synthetic sapphire, highly scratch resistant. Standard on most mechanical watches above entry level.
Mineral crystal
Hardened glass. Cheaper than sapphire and more prone to scratches; Seiko's version is called Hardlex.
Acrylic (Hesalite)
Plastic crystal. It scratches easily but polishes out and doesn't shatter, which is why the Omega Speedmaster Moonwatch still uses it.
Indices
The hour markers on the dial. Applied indices are separate pieces fixed to the dial; printed ones are painted on.
Lume
Luminous paint on the hands and markers that glows in the dark after charging in light.
Cyclops
A magnifying lens on the crystal over the date window, a Rolex signature.
Sub-dial
A small dial inside the main dial, used for running seconds or chronograph counters.

Movements

Movement (caliber)
The engine of the watch. Caliber is the maker's name or number for a particular movement design. Movements in the almanac
Automatic
A mechanical movement wound by a rotor that spins as your wrist moves. It keeps running as long as you wear it.
Manual (hand-wound)
A mechanical movement you wind by turning the crown, usually daily. Often thinner than an automatic because there's no rotor.
Quartz
A battery-powered movement regulated by a vibrating quartz crystal. Far more accurate than mechanical and cheaper to make.
Solar
A quartz movement charged by light through the dial, so it rarely needs a battery change. Citizen calls it Eco-Drive.
Spring Drive
Seiko's hybrid: a mainspring powers the watch, and a quartz-regulated electromagnetic brake controls its speed, giving a smoothly gliding seconds hand.
In-house
A movement designed and made by the watch brand itself, as opposed to one bought from a supplier.
ETA, Sellita, Miyota, Seiko (NH35)
The main suppliers of movements to other brands. ETA and Sellita are Swiss; Miyota (Citizen) and Seiko's NH-series are Japanese.
Power reserve
How long a fully wound watch runs before stopping, usually 38 to 80 hours for modern automatics.
Beat rate (vph)
How many times the balance swings per hour. 28,800 vph (4 Hz) is common; higher rates sweep more smoothly.
Jewels
Synthetic rubies used as low-friction bearings in a mechanical movement. More isn't necessarily better.
Hacking
Seconds hand stops when you pull out the crown, so you can set the time to the second.
Chronometer (COSC)
A movement certified accurate by the Swiss testing body COSC: within -4 to +6 seconds a day.
Master Chronometer (METAS)
Omega's stricter certification, tested in the cased watch for accuracy (0 to +5 seconds a day) and resistance to magnetism.

Complications

Complication
Any function beyond hours, minutes and seconds.
Chronograph
A stopwatch built into the watch, started and stopped with pushers on the side of the case.
GMT
A fourth hand that circles the dial once every 24 hours to show a second time zone, read against a 24-hour bezel or scale.
Moonphase
A display of the moon's current phase through a window in the dial.
Tourbillon
A rotating cage around the escapement, invented to offset the effect of gravity on pocket watches. Today mostly a showcase of craft.
Day-date
A watch that shows both the date and the day of the week.

Straps and bracelets

Spring bar
The spring-loaded pin that holds a strap between the lugs. A spring bar tool removes it.
Integrated bracelet
A bracelet designed as part of the case shape, as on the Royal Oak or Tissot PRX. Usually it takes only its own bracelet. Resizing a bracelet
Micro-adjust
Extra holes or a sliding extension in the clasp for small changes in bracelet length.
Deployant clasp
A folding clasp, on bracelets and some leather straps, that opens out and folds closed instead of using a buckle.
NATO strap
A single-piece nylon strap that threads under the watch through both spring bars. NATO straps

Specs and the market

Water resistance
A test pressure rating, not a safe diving depth. Roughly: 30–50 m for splashes, 100 m for swimming, 200 m and up for diving.
Reference number
The maker's code for an exact model, dial and material. Two watches with the same name can have different references and sizes.
Microbrand
A small independent watch company, usually selling direct online. Best microbrand watches
Homage
A watch closely modeled on a famous design, such as a Submariner, usually from a lower-priced brand.
Grey market
New watches sold by dealers who aren't authorized by the brand, often at a discount but without the brand's warranty from that seller.
Tool watch
A watch designed for a job (diving, flying, field use) rather than for dress.