Drop-lock occurs at the instant when the drop ends and the escape wheel tooth contacts the pallet stone. It is measured as the distance between the corner of the pallet stone and the heel of the escape wheel tooth at the instant the escape wheel tooth contacts the pallet stone. Drop-lock, like total lock, is often expressed in terms of a fraction of the length of the impulse face of the pallet stone. Drop-lock must occur on the locking face of the pallet stone. Drop-lock must be equal on the entry and exit stones; or stated another way: drop-lock must occur at the same distance from the corner on both the entry and exit stones. The drop must not end on the impulse face of the pallet stone because that could cause the watch to have poor amplitude.