Beat error

Beat error is the difference in timing between the two halves of the balance's swing, showing how evenly the movement ticks.

A small beat error is normal; a large one can make timekeeping less stable and suggests the movement needs adjusting, usually a routine job for a watchmaker.

Beat error appears alongside rate and amplitude on a timegrapher, and it's one more quick clue to a movement's condition when you're pricing a watch.

Readings shift with position and wind, so compare a watch across a couple of positions before drawing conclusions from a single number.