and entomology, as groups of related taxa in these fields tend to be very large. However, they have also been used to classify non-organisms, such as birds nests, and in non-biological sciences such as geology. Similar methods have also been used in computer science.

A user of a key selects from a series of choices, representing mutually exclusive features of the specimen, with the aim to arrive at the sole remaining identity from the group of taxa. Each step in the key employs a character: a distinguishing