What are central seconds and sub-seconds?

　　What are central seconds and sub-seconds? What is the difference between them? If you are interested, read on to find out.

　　Definition of central seconds

　　There are 3 main hands on a watch: the hour hand, minute hand, and seconds hand. To make the hands as easy to read as possible, these 3 hands need to be placed at the centre of the dial (or the centroid for non-circular dials). When the hour, minute, and seconds hands are all positioned at the centre, this dial design is known as central seconds.

　　Definition of sub-seconds

　　Configuring the hour and minute hands as large hands, while carving out a separate seconds sub-dial elsewhere on the dial with a small seconds hand (such as Panerai's signature torpedo small seconds), is known as a sub-seconds hand design.

　　Difference between central seconds and sub-seconds

　　The most obvious difference between central seconds and sub-seconds lies in reading the seconds. Because central seconds display every single second clearly, they allow you to read time to at least the nearest 1 second, with some watches offering an accuracy of 1/4 to 1/10 seconds or more. In contrast, sub-seconds are much harder to read down to the exact second, generally providing an accuracy of around 5 seconds. This is why the chronograph seconds hands on many stopwatches still use the main dial for reading times.

　　When mechanical watches were first invented, the hands were not coaxial; instead, there were three separate sub-dials, with the hour, minute, and seconds hands each having their own dial for independent reading (this was the original regulator design). Later, to reduce the size of mechanical watches, the concept of placing the hour, minute, and seconds hands on a single axis was proposed. Given the technology of the time, this was a highly demanding process, because having 2 coaxial hands required the outer shaft to be hollow, with a thinner shaft passing through the middle, whilst maintaining a clearance between the two shafts due to their differing rotational speeds. The hand shafts of a watch are already very thin, measuring no more than 1-2 mm. One can only imagine how difficult it was to drill a uniform hole through such a tiny metal rod. Therefore, managing to place the hour and minute hands on a single axis was already a remarkable achievement; coaxially mounting the seconds hand as well was simply too demanding for mechanical pocket watches of that era. As a result, older pocket watches mostly featured sub-seconds designs, with a separate small seconds sub-dial and a main dial with coaxial hour and minute hands.

　　Later, with advancements in technology, mechanical watches transitioned from pocket watches to wristwatches, demanding smaller sizes and greater precision. Coupled with the maturation of metal processing techniques such as stamping and extension, hollow shaft technology became increasingly refined, and hollow shafts within 1 mm became a reality. Once the technical hurdle of central seconds watches was overcome, they essentially became the mainstream design for wristwatches. This explains why central seconds watches are currently mainstream while sub-seconds watches are considered vintage.