A watch called “au repère” is a watch whose resonator is perfectly centered when it is in neutral.

If we take the example of a Swiss lever escapement (in a straight line), the  ellipse must be perfectly centered in the fork of the  pallets, which will itself be perfectly aligned with the line where the center of the escape-wheel, the pivot of the lever axis and the center of the balance wheel are located.

For a person who has a musical ear, a timepiece that is well on the mark will be heard by its regular sound. If it gallops it is not on the mark. To get a more precise graphic idea the curve of a timing-machine will help where the parallel lines of a watch that is not on the mark will end in a single line of a perfectly on the mark timepiece.

The principle of the reference mark is obviously also valid for the pendulum of a wall clock which would also gallop if the clock is not straight. Although important in principle, the repère does not have such an important impact on the final accuracy of a timepiece.