Subdials are busting out all over this year as manufacturers increase their offerings of chronographs and other multifunction watches. What do all those whirling doodads do?

1. What is a subdial?

A subdial, or subsidiary dial, is a small dial placed inside the main dial on a watch face. Watches can have as many as four of them. Subdials, also called auxiliary dials, give information not provided by the main watch dial. They are a common features of multifunction watches such as chronographs, alarm watches, dual-time-zone watches and calendar watches.

2. What kind of information do they give?

Many kinds. Chronograph watches, those with a stopwatch function, use subdials to keep track of seconds and of elapsed minutes and hours. Calendar watches often have subdials with pointers indicating the month, date and sometimes day of the week. A moon phase subdial shows what phase the moon is in by means of a disk that rotates beneath a small aperture. On the disk are painted two full moon faces. As the day of the month pass, the painted moon (only one is visible at a time) either waxes or wanes in synchrony with the phase of the real moon.

In a mechanical watch, one that is powered by a mainspring rather than a battery, a subdial can also be used to show how much power remains before the watch stops running. Such a subdial is called a “power reserve indicator”. (Quartz watches also sometimes have similar devices, showing how much power is left in the battery or energy cell. These devices, however, are usually incorporated into the watch’s main dial rather than a separate subdial.)

On an alarm watch, one that rings at a specified time, a subdial is sometimes used to set the alarm. Dual-time-zone watches often have subdials that show the time in another time zone. Sometimes these subdials express the time in military fashion, on a 24-hour, rather than 12-hour, basis. (Dual-time-zone watches are sometimes called GMT watches - short for Greenwich mean time- because their second-time-zone subdials can be set to the local time at the prime meridian, which passes through Greenwich, England. Of course, the wearer can set the second-time-zone indicator to give the time in any time zone in the world, not just at the prime meridian.)

Some subdials do double-duty, using two scales and two hands. For example, one hand might point to the day of the week while the other keeps track of the seconds. Or a moon phase subdial might also have a pointer that shows the date. Note that not all subdials are round. Some are designed in what is called the bras en l’air /French for "arm in the air) design, in which the hand moves through an arc rather than in a circle. Power-reserve indicators are sometimes designed in this way; as sometimes are date indicators.

3. Explain the various subdials on a chronograph

One subdial is used for the seconds hand. In most models, the seconds subdial shows continuously running seconds. The stop-start seconds hand, controlled by the chronograph button, is placed at the center of the main dial. This is done for reasons of readability- the main dial is easier to see than a small subdial. In some watches, those with tachymeter and telemeter scales (used for measuring speed and distance, respectively), placing the chronograph seconds hand in the center serves another purpose as well. These scales are printed along the circumference of the main watch dial, and can only be used if the elapsed-seconds hand is in the middle of the watch.

Some chronographs also have subdials that show the fraction of a second - most often 1/10s of a second. Chronographs often have other types of subdials as well, called “counters”, “registers”, or “totalizers,” which keep track of the minutes and hours that have elapsed since the wearer pushed the chronograph button. Most minute registers are graduated in 30 segments; most hour registers in 12 segments. If the wearer wants to measure a longer period of time than the subdial permits, say 45 minutes, he adds the time recorded on the minute register’s first revolution, 30 minutes, to the time that has elapsed on its second revolution, 15 minutes. The reason most counter only go up to 30 minutes is that they are small. If they were marked in 60 increments, a full hour, they would be difficult to read.

Some chronographs use a subdial to show the “real” time, the current hours and minutes, while the large main dial is dedicated entirely to the chronograph function.

4. How do you set subdials?

In a chronograph, the counter start and stop recording time when you push the chronograph button, the same button starts and stops the chronograph seconds hand. When you want to reset all the dials to zero, you push another button. A system of levers underneath the watch daily returns all the subdial hands to their original position simultaneously. Other types of subdials - calendars and alarms, for instance - are set using the watch crown or a separate button on the watch case.

5. How can you tell what a subdial is used for?

It’s not as complicated as it seems. If a subdial has a 60 at the top, it’s probably a continuously running seconds hand (if the watch is working, this hand will be moving). But a few subdials with 60 at the top are actually 60-minute counters.

If the subdial has a 30 at the top, its most likely a 30-minute counter. If it has a 12 at the top, it’s probably a 12-hour counter (although if the watch is a dual-time-zone model, its probably a second-time-zone indicator). If it has a 10 at the top, it probably measures 1/10th of a second.

Date subdials have a 31 at the top for the maximum number of days in a month. Moon phase subdials are obvious - they have a picture of a moon showing through the aperture - as are month and day-of-the-week subdials, which are labeled either Jan. through Dec. or Sun. through Sat.

6. Some non-chronograph watches have their seconds hand set in a subdial instead of in the center of the dial. Why?

It’s purely for cosmetic reasons. A main dial with just two hands is sometimes easier to read than one with three, and the subdial give the watch added visual interest.

7. Why do so many watches have subdials these days?

There are two reasons. First, multifunction watches are extremely popular, and subdials are useful, and often necessary, in displaying the various types of data they measure. Second, subdials give a watch a high-tech and/or sporty look - which is extremely fashionable nowadays. That’s why some watches that don’t really need subdials -ones that simply show the day of the month, for example, or incorporate a seconds hand - have them anyway. Subdials have become extraordinarily important in watch design.

8.Are they so fashionable because they’re new?

Not by a long shot - hundreds of year, in fact. Subdials can be seen on some of the earliest watches in existence, dating back to the 17th century. In those days, watches weren’t accurate enough to count seconds (they could barely keep track of hours), so subdials were used to display the day, date and moon phase.

Source: American Time