All watches (except extremely fine ones) lose to a certain extent in the pocket and go faster when lying horizontally, the difference varying from thirty seconds to one minute per day. It is caused by the more free vibration of the balance when poised on the end of one pivot only than when resting on the sides of two pivots, as it does when in the pocket. If the difference in a watch exceeds one minute per day, most likely the balance is not truly poised - that is, it is heavy at one point of the rim - and acts more or less as a pendulum when the watch is vertical. To remedy it, remove the balance and take off the hairspring. Then place the balance with its pivots resting on two finely polished straightedges, on which it can roll freely and be tested for poise, any fault being corrected by means of the screws in the rim (if it has them) or by filing, if it is a plain balance. Poising tools are sold for this especial purpose.