A balance spring system of a horology movement, comprising: a balance spring (1); a movement blank; a first element (1c) for indexing the position of an outer end (10) of the balance spring relative to the blank; and a unit (OL) for connection of the outer end (10) of the balance spring to the movement blank, the first indexing element (1c) being designed such as to be displaceable relative to the connection unit (OL).