Keys are devices bearing a series of contrasting
or contradictory statements (leads),
requiring an identifier to make comparisons
and select the statements that match the material to be identified.
A commonly used key is the Dichotomous key.
A dichotomous key offers only two choices at every step.
Such keys can be presented in either indented or bracketed forms.
The advantage of using a key is that it is fast,
because it directs the identifier to check definitive characteristics.
The disadvantage of a key is that it tends to be rigid,
and would become useless when the stated characters are not available.