As one can imagine, constructing a chronograph is fraught with the potential for inadvertently stopping or damaging the entire movement. For example, if the reset mechanism was somehow activated while the chronograph was running, large destructive forces would be applied to the entire movement train, which would at the very least jam the movement, and would likely destroy several components.

The column wheel was one of the successful early designs to ensure that none of the above happened. If you can visualize the top of a castle turret, with tooth-like battlements, this is what a column wheel looks like. The pushers that control the chronograph rotate this wheel, and the various parts of the chronograph are controlled by fingers that fall into and out of the spaces between the teeth. This ensures that each of the chronograph parts is coordinated properly.

Because producing and finishing a column wheel is labor-intensive, a simpler, easier method of producing chronographs was needed to keep this complication from appearing only in very expensive watches. The most common method is to use an oscillating pinion to coordinate the chronograph start, stop, and reset (i.e. a cam shaped device that rotates back and forth as the various functions are activated.) This allows for reliable operation without nearly as much fiddling and hand adjustment as a column wheel requires, and the pinion can be built using pieces of stamped metal.

These days, relatively few column wheel designs are still being produced - the Zenith El Primero, some Lemania and F. Piguet movements, and (interestingly) a F. Piguet rattrapante mecaquartz are examples. The most common movements, such as the Valjoux 7750 and various Lemania movements (1874, 5100), are not of column wheel design.

(used with permission of TimeZone)