To obtain more changes than available in the plain course, a "conductor" makes a "call" directing the ringers to make a slight variation in the course. (The most common calls are called "bobs" and "singles".) These variations usually last only one change, but cause two or more ringers to swap their paths, whereupon they continue with the normal pattern. By introducing such calls appropriately, repetition can be avoided, with the peal remaining "true" over a large number of changes. For example, an extent in a minor method is 720 (6!) changes, so would require 12 repetitions of the plain course shown.