Beat

The hairspring has to be installed rotated with it's stud in the right place. The hairspring collet is friction fit to the staff, and so it can be turned (with the right tool). The stud, at the outside of the spring's coils, is held fixed by the balance cock when assembled. The stud has to be positioned such that the spring's at-rest position places the roller jewel dead center in the also dead centered fork.

Having set the roller table so that the jewel is 90 degrees to the arms of the balance, the balance wheel arms should be perpendicular to a line through the balance jewels, the pallet fork jewels and the escape wheel jewels.

If this is not right, then the balance wheel, when running, will turn more to one side than the other. A watch like this is said to be "out of beat". It will keep time poorly, and may even stop.