Programmable locking cylinder

The locking cylinder (1) has a stator (6) and a rotor (5) which can rotate in the stator and has a key opening into which a key (10) can be inserted. Said locking cylinder (1) has a multiplicity of tumbler (2) and mating-tumbler (3) pairs, wherein each of the tumblers (2) has a first part (2a) and a second part (2b) which are connected to one another, for example, with a press fit. In the method for programming a locking cylinder, the locking cylinder (1) has at least one two-part tumbler (2), having parts (2a, 2b) which are connected to one another with a press fit. In the method, when a key (10) is introduced into the key opening a length (L), effective in terms of locking, of the tumbler (2) is changed by the first part (2a) and the second part (2b) sliding one into the other or moving apart from one another, in particular is reduced by them sliding one into the other.