dichotomous keys – a method used to identify organisms by following a series of paired choices based on observable characteristics

Note: "Dichotomous" means "divided into two parts" a reference to the number of choices available at each stage.

To remember the meaning of dichotomous keys, use the following mnemonic:

The dyed cotton hippopotamus with keys (dichotomous keys) had to choose between two doors.

By systematically working through the binary choices offered by a dichotomous key and selecting the option that matches the organism being identified, the user is guided step by step towards the correct identification. This is particularly useful for fieldwork and laboratory identification because they focus on easily observable features like size, shape, colour, or number of body parts. The most unambiguous observations work best when using dichotomous keys. 

The following is an example of dichotomous keys being used to determine which sub-species a particular vertebrate belongs to:

Dichotomous keys can also be used to identify plants using the same process of judging easily observable features.