To poise a watch balance, first remove the hairspring and brush the balance and pivots clean. Then place the balance on the parallel edges of a poising tool or in a pair of callipers and allow it to settle. It will always rest with the heavy part downwards. Gentle tapping of the callipers assists it to settle. With a plain balance, lighten the heavy part by filing the inner under edge of the rim; with a balance having screws, reduce a screw lightly or add washers to the light screws.