The fundamental difference between chronographs and standard watches is that a chronograph measures specific time intervals through a user-activated command. At the same time, it allows the normal flow of time to be measured. These two functions, which occur together, distinguish a chronograph from a stopwatch, which instead only measures time intervals, but cannot show the time.
There are two main types of mechanical chronograph systems. The first was integrated into the movement, and was based on the operation of a particular wheel, called a column wheel, which made chronograph movements such as engagements and stops of the chronographic function smoother and more precise.
The second was developed later. It was a simplification of this complex and costly system, based on a system of levers and cams that avoided the need for a column wheel. More, it could be built in an add-on module that companies can superimpose on a traditional movement. The most famous manufacturer of these modules is Dubois-Depraz, a specialized company that has worked with all the big names in the Swiss watchmaking industry. Incidentally, the first automatic watch movement, called Chronomatic, was developed in 1969 by a consortium of companies where Dubois-Depraz was an influent member.
In a mechanical watch movement, the chronograph function requires two different wheel trains, precisely because the two processes must be linked but have to stay independent of each other. Obviously, in a digital quartz movement, this is simpler since everything happens thanks to the programming of a microchip that controls the various functions of the timepiece, and the results are displayed on an LCD screen.
Often chronographs have particular scales on the dial or the bezel, which were used to make calculations when the world did not have available tools such as computers. It was typical, in fact, to use pilot's chronographs to calculate the route of airplanes or racer watches to calculate the speed of a vehicle. And although these scales are no longer used today, they can be found in vintage watches and as a stylish design element in many modern timepieces.