Split-seconds chronograph allows to measure intermediate times. This is made possible by two chronograph seconds hands superimposed on each other. Normally the two hands are slightly different at the level of their counterweight. This is to differentiate the first hand stopped from the second. One can sometimes also find two superimposed hands on one or more counters, so the measurement of intermediate time goes beyond the minute.
The most prestigious split-seconds chronographs are equipped with two column wheels, one column wheel for the traditional chronograph and an additional column wheel for the split-seconds which will also have an additional pusher to activate the split-seconds. On a watch with a sapphire case back, these two column wheels can be seen working independently.
Until the middle of the 20th century all split-second chronographs were equipped with two column wheels. After that chronograph modules that are placed between a standard movement and the dial arrived. This last version is of more common quality but also makes this complication more accessible.