A sub-dial is a small dial set inside the main dial of a watch. Also called a register or subsidiary dial, it displays extra information — like elapsed time, running seconds, or the date — separate from the main hour and minute hands.
What sub-dials show
The most common use is on a chronograph, where sub-dials count the elapsed minutes and hours you've timed. You'll also see a small running-seconds sub-dial (common on hand-wound watches), a 24-hour indicator, a day or date, or a power reserve gauge. Each sub-dial keeps one job from cluttering the main time display.
Reading a chronograph's sub-dials
On a typical chronograph, the large center-mounted hand counts the seconds you're timing, while the sub-dials tally the minutes and hours. The small running-seconds sub-dial — the one always ticking — simply tells you the watch is running. It's easy to confuse the two, so remember: the center hand is the stopwatch, and the little constantly moving dial is just the watch keeping time.
Sub-dials and layout
Watch fans describe layouts by how many sub-dials sit on the dial and where. A tri-compax arrangement has three. Their placement is a big part of a watch's look and balance, which is why designers obsess over spacing and symmetry.