Identification keys: The devices for identifying the samples from a set of known taxa, which contains a tree- structure where each node corresponds to the diagnostic question of the kind 'which one of a named set of attributes does the specimen to be recognized possess?' The result determines the branch of the tree to follow and hence the next diagnostic question leading eventually to the correct identification. Many times there are only two attributes, concerning the presence and absence of the particular character or the response to a binary test, but multi-state characters or the tests are permissible.