A timepiece balance spring made of micromachinable material including a plurality of stages, each forming a spiral wound spring, parallel to each other and all attached to a common axial collet or to the same balance staff, each stage including, at its respective outer end, its own attachment to a balance spring stud which are independent of those of the other stages, the attachment including a position adjustment with respect to a balance spring stud which are also independent of those of the other stages, the attachment and the position adjustment together forming built-in isochronism correction mechanism for the balance spring, and the position adjustment including a plurality of discrete position adjustment positions including housings each arranged to retain a stud.