Subdials are the mini-dials that sit on the watch face or dial. Also known as auxiliary dials, subdials serve different functions—like tracking lapsed seconds, minutes, and hours, the phases of the moon, a second time zone —across mechanical and specialty watches like chronographs, calendars, and GMT watches.

Here we break down the different purposes subdials can serve on particular watches, including the chronograph and moon phase.

Chronograph Subdials

The most common subdials are found on chronograph watches. In the most basic terms, a chronograph is a stopwatch. By pressing the pushers on either side of the watch case, the wearer can activate the chronograph seconds hand or stop watch functionality.

A chronograph’s subdials are sometimes referred to as “registers” that keep track of the total elapsed minutes and hours timed with the chronograph. Another subdial also measures the seconds, down to 1/10th of a second for added accuracy.

The subdials are meant to improve the overall readability of the measured elapsed time by breaking down the seconds, minutes, and hours separately. By adding up the figures you read in the subdial, you can calculate how much time as passed since you started the chronograph.

Power Reserve Indicator Subdial

Some mechanical watches (which rely on a mainspring to power the watch) will have a subdial that reads out how much stored energy the watch has left before it stops running.

GMT Subdial

Popularized during the 1950s when commercial air travel was a growing mode of transportation, GMT watches allow you to keep track of a second time zone. Typically, the second time zone is set and read via a rotating GMT bezel and a GMT hand. However, select GMT watches will actually feature a GMT subdial were the second time is displayed instead.

Moon Phase Subdial

Unlike the other subdials on the list which feature numbers, the moon phase subdial features a depiction of the moon through an aperture (or window) that tracks the phase of the moon (i.e. new, waxing, crescent, full, etc.)

Day of the Week Subdial

Another less traditionally thought of but still classified as a subdial is the day of the week subdial. Similar to the moon phase, the day of the week appears through an aperture on the main dial and reads out the respective day. One watch model that prominently features a day of the week subdial is the Rolex Day Date.