Take a glance at any tool watch and one of the first things you’ll notice is the bezel. At a glance, you’ll be able to tell whether you’re looking at a diver, a GMT or even a watch that’s built to measure distance. Bezel scales are far more than decorative, though. They’re matched to specific complications and provide a clever, tactile extension of a watch’s functionality. Some are simple and straight forward to use while others look like something that requires an engineering degree to operate. Here’s a look at 12 different kinds of bezels.

You probably already know but just in case, a bezel is the external ring that surrounds a watch’s crystal. Some watches also have a rotating internal bezel tucked under the crystal that is usually controlled by a dedicated crown. Bezels can be constructed out of any materials but are often steel with a ceramic insert for durability and scratch resistance. Bezels can be fixed or free to rotate in one direction or both. For many watch lovers, the pleasing resistance and click of a rotating bezel is one of the hidden benefits of owning a tool watch.

Count Up Bezel
Dive watches are the most popular tool watches, which makes the count up bezel, also called a dive bezel, the most common ring on a watch. Dive bezels only rotate counterclockwise so that the displayed duration of a dive cannot be accidentally shortened by an inadvertent touch. The 60-minute scale is broken down into 10-minute increments and usually shows single minute markers for 0-15 minutes that are used to time a safe ascent to the surface. You’ll find this bezel on any pre-owned Rolex Submariner, the Tudor Black Bay 58, Mido Ocean Star Diver 600 and lots of choices from Seiko, Bell & Ross and others.

$31,800

Count Down Bezel
Unlike a count up bezel, count down bezels rotate both directions and are a convenient way to time anything with a set duration. I use the one on my watch frequently when I’m cooking. To use it, just turn the bezel so the amount of time you want to count is lined up with the minute hand. When the minute hand hits the triangle marker, time is up.

Bidirectional Rotating 24-hour GMT Bezel
Travel watches often feature a bidirectional rotating 24-hour GMT bezel. It shows a second time zone when aligned with the GMT hand on watches like a pre-owned Rolex GMT Master II or Oris Big Crown ProPilot GMT. Watches equipped with a 12-hour rotating bezel can also show a second time zone by positioning the triangle marker above the second time zone’s hour and reading the time using the bezel’s markers. World Time bezels show cities from each of the world’s 24 primary time zones. The Girard Perregaux 1966 WW.TC is a great example. Its internal bezel adjusts by turning the second crown at 9 o’clock.

$2,600

Fixed Tachymeter Bezel
Chronographs like the TAG Heuer Carrera and Zenith Chronomaster feature a fixed tachymeter bezel that‘s used to measure units per hour. The most common application is to calculate speed. To do so, just start the chronograph then stop it when you’ve covered one mile. The point on the bezel where the chrono hand points shows miles per hour. It is calculated by dividing the elapsed time by 3,600, the number of seconds in an hour.

Price on Request

Internal Compass Bezel
The Seiko Prospex SPB210 features an internal compass bezel that’s controlled by the second crown at 4 o’clock. Compass bezels typically rotate smoothly in both directions for easy adjustments. Setting one can be done with a little Boy Scout knowledge. Align the hour hand with the position of the sun then rotate the bezel so south is halfway between the hour hand and 12 o’clock. This only works during Standard Time in the Northern Hemisphere so if it’s Daylight Savings Time, you will need to adjust the position of the hour hand back by one hour.

Price on Request

Telemeter Bezels
Telemeter bezels calculate distance with a scale calibrated to the speed of sound. They function similarly to a tachymeter bezel since the distance corresponds to the position of the chronometer hand. A good use is to figure out how far away a lightning strike is. Just start the chronograph when you see the flash and stop it when you hear the thunder. The chrono hand points to the distance in either miles or kilometers, depending on how the watch is configured.

Pulsometer Bezel
Doctor’s watches usually have a pulsometer bezel that provides a quick display of heartrate in beats per minute. The Longines Heritage Pulsometer chronograph is a great example which is used by stopping the chrono after feeling 30 pulses to display the heart rate.

$2,325

Slide Rule Bezel
The slide rule bezel is one of the more intimidating and mysterious scales. It can be used to perform the kinds of mathematical calculations pilots might need but can also be used for more mundane things like calculating a tip or dividing up a bill. Some slide rule bezels have markings for unit conversion, speed calculation and compass readings, too. The Breitling Navitimer is one of the most well-known watches featuring this bezel, which can also be found on the Seiko Flightmaster Chronograph.

$13,875

Combination Bezels
Though not a slide rule, the Hamilton Khaki Aviation X-wind combines several bezels to display information specifically for aviators. It has three bezels — one external and two internal, each controlled by its own crown. The external bezel shows airspeed while other two display windspeed and wind direction. Together with a graph on the caseback, the bezels allow pilots to calculate bearings corrections to offset crosswinds. And for all these years, I thought the Navitimer was the most daunting watch out there.

$1,595