What's a key?

In the environmental studies, a key refers to a guide for the classification of organisms based on its specific and unique characteristics.

Keys might be dichotomus or polichotomus depending of how many variables do each one use for each branch of the classification.

The use keys make easier to place each new organism in a certain area and to understand the specifications of each species and how they differ form other. It might also make reference to evolution processes that species have underwent.