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

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.

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.

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