What is a bezel on a watch?

The bezel is the ring that surrounds the crystal (the glass) on top of a watch case. It holds the crystal in place and protects it, and on many watches it is also a tool: a rotating dive bezel times a dive, a 24-hour GMT bezel shows another time zone, and a tachymeter bezel measures speed.

What it is	The ring around the watch crystal
Main jobs	Hold the crystal, protect it, add a function
Dive bezel	Turns one way only, 60 minutes
GMT bezel	24 hours, for a second time zone
Tachymeter	Measures speed with a chronograph
Common materials	Steel, aluminum, ceramic, gold

Types of watch bezel

Bezel	What it does	Found on
Fixed / plain	Decorative and protective only	Dress watches, Datejust
Fluted	Ridged decoration that catches the light; originally functional for screwing the bezel down	Rolex Datejust and Day-Date
Dive (unidirectional)	60-minute scale that turns only counter-clockwise, to track elapsed time underwater	Submariner, Seamaster, most divers
GMT (24-hour)	24-hour scale read with a GMT hand for a second or third time zone	GMT watches
Tachymeter	Converts elapsed seconds into speed per hour	Speedmaster, Daytona, Carrera
Countdown	Numbers run backwards to count down to an event	Regatta and sports watches
Slide rule	Two rotating scales for multiplication, fuel and distance	Breitling Navitimer
Compass	Cardinal points for finding direction with the sun	Field and outdoor watches
Gem-set	Diamonds or colored stones	Jewelry and luxury models

How to use a dive bezel

At the start of something you want to time, a dive, parking meter or pasta, turn the bezel so its triangle (zero) lines up with the minute hand. As time passes, the minute hand points to the minutes elapsed on the bezel. Dive bezels turn only one way, counter-clockwise, so if one is knocked by accident it can only show more time elapsed, never less: the safe mistake when your air supply depends on it. For timing on screen, try our stopwatch.

How to use a tachymeter bezel

Start the chronograph as you pass one marker (a mile post, for example) and stop it at the next one, one unit of distance later. The number on the tachymeter where the seconds hand stops is your average speed in units per hour. If it takes 45 seconds to cover a mile, the hand points to 80: you're traveling at 80 mph. It works for any unit: kilometers, items made on a production line, and so on.

Bezel materials: ceramic vs aluminum

Aluminum inserts are light and cheap but scratch and fade over the years; collectors love the faded "ghost" look on vintage pieces.
Ceramic (Rolex calls its version Cerachrom) is almost scratch-proof and its color never fades in sunlight.
Steel, gold and platinum bezels are often polished or fluted.
Sapphire and Bakelite appear on some designs; the first Rolex GMT-Master's Bakelite bezel cracked easily and was replaced within a couple of years.