1) Two or more classes of alternative responses. Concurrent operants may be compatible (as when a rat simultaneously presses one lever with its left paw and another with its right) or incompatible (as when the pigeon, having only one beak, pecks only one of two keys at a time) as long as the organism can en-dt either or can change over from one to the other at any time (occasionally, responding and not responding are treated as concurrent operants). Discriminated operants also may be concurrent if at any time the organism has an opportunity to produce the stimuli that occasion them. For example, in a changeover-key procedure, a pigeon changes the stimuli and their associated schedules on one key (the main key) by pecking a second key (the changeover key). In this case, two concurrent schedules operate on one key, and changeovers are an explicit class of responses on the second key. See also changeover, concurrent schedules, preference.

2) Two or more responses, of different topography at least with respect to locus, capable of being executed with little mutual interference at the same time or in rapid alternation, under the control of separate programming devices (e.g., responses to two keys present at the same time under separate schedules).