The collet, which can be made in a single piece and at the same time as the balance-spring, is formed by a plate (1) which includes an aperture (3) for attachment to a balance staff (5) and has an asymmetrical contour (9) which follows at a substantially constant distance d the contour of the first coil (11) of the inner terminal curve. The plate can further include recesses (13) for repositioning the centre of gravity g on the balance staff (5).