The behaviour an organism can emit, in the sense that the behaviour exists at a nonzero level, has been shaped, or, if extinguished, may be rapidly reinstated. The organism need not be engaging in the behaviour for it to be in its repertoire (e.g., a rat that has learned a maze has maze-running behaviour in its repertoire even when not in the maze). To the extent that some responses in it are more likely than others, a repertoire consists of a hierarchy; operant procedures modify the relative positions of responses in the hierarchy.