A watch with central seconds positions the second indication at the central axis of the watch dial, usually coaxial with the hour and minute hands. This is also known as center seconds or sweep seconds.
Central seconds was an important complication developed for the smaller dials of wrist watches in the first half of the 20th century. Pocket watches originally had what is known today as small seconds, that is the seconds indicator resided in a subdial. This arrangement was so typical that the gear train came to be named after it: The Minute wheel is known even today as the center wheel, even though it is usually no longer located at the center of the watch thanks to the advent of direct central seconds (see below).
Beginning in the 1940's, as watches became popular with men generally and doctors and other professionals in particular, demand grew for a larger swept area for the seconds hand. This allowed more precise reading of the time down to the seconds. Due to the added complexity of this arrangements, central seconds became a valued complication and selling point for fine watches. Today, this arrangement has become so popular that it is now considered the typical arrangements, with subdial small seconds becoming the notable complication.
The first watches with indirect central seconds appeared in the 1910's, but it was not until the 1940's that this became popular. American watch companies first delivered central seconds watches in the 1950's. Zenith introduced the now-standard direct central seconds complication in 1948 with their calibre 133. This concept was rapidly adopted in new calibres across the industry.
It is difficult to drive three hands at the center of a watch, requiring a rearrangement of the traditional gear train and added wheels.
Indirect central seconds
One way to drive a central seconds hand without disturbing the placement of the center wheel is to drive it indirectly off the fourth wheel. This arrangement, called indirect central seconds has the benefit of leaving much of the watch works in their traditional locations. But it moves the seconds hand outside of the power flow in the gear train, causing issues with visible "flutter" of the seconds hand. It also adds complexity and thickness to the movement, with an extra layer of gears below the central pinion.
Direct central seconds
Since it would be difficult to detect flutter in slow-moving minute and hour hands, most movements today drive the seconds hand directly off the fourth wheel and the hour and minute hands indirectly through a coaxial intermediate wheel and cannon pinion. This rearrangement of the gear train results in a thinner and less-complex movement (though still often more complex than small seconds). It also eliminates flutter of the seconds hand since it is in the power flow.
One notable approach to the design of direct central seconds came from Patek Philippe. Their 12-SC movement left the center wheel in the center of the dial but placed the fourth wheel directly under it, with a co-axial shaft running through a tube all the way to the dial. This allowed both the hour/minute hands and the seconds hand to be in the power flow and resulted in a more compact yet still traditional design. But this was not long continued.