Describe how a dichotomous key is used.

A dichotomous key is a special document that allows the reader to 'key out' an organism to some level, sometimes species, sometimes a level above species.

A dichotomous key is a method for determining the identity of something (like the name of a butterfly, a plant, a lichen, or a rock) by going through a series of choices that leads the user to the correct name of the item. Dichotomous means "divided in two parts".