The specific design and construction of the key varies, depending upon the organism operating it. The term key has come to be a generic term, synonymous with manipulanda, foot treadle, or lever. A performance frequently recorded in the performance of a monkey or a chimpanzee is that of pressing a toggle switch similar to the key used on a telephone switchboard. All these devices have the advantage that the relationship between the performances and their effect on the food dispenser may be objectively and accurately specified on automatic recorders. Experiments with performances which do not involve the operation of a switch (such as head raising in the pigeon) are much more difficult to define and require personal judgement as to when the performance conforms to a criterion which defines a class of performances objectively.