A change is a means for getting from one row to another. It works by swapping over pairs of bells, and no bell may move more than one place.
The normal way of representing a change is by place notation; a single change is represented by a series of numbers which each correspond to a place being made; for example, 12 means that all bells swap, apart from the 1 and the 2, which stay in the same place. If all the bells swap, the place notation is X.