Key is an important tool to study a taxonomic category. A key is composed of two contrasting characters. These characters generally occur in pairs and such a pair is called a couplet.
While studying a key; either of the contrasting characters is rejected or accepted. Thus, a choice is made between the two contrasting characters in a key. Each statement in the key is called a lead. It is important to note that a key is usually analytical in nature. Moreover, separate keys need to be used for different taxonomic categories.
Keys help in step-by-step identification of a particular organism. For example; presence or absence of notochord gives us a clue whether the organism belongs to vertebrate or invertebrate. Similarly, presence of absence of lungs tells us if the organism belongs to tetrapoda or pisces.