dichotomous key

In biology, a practical method used to identify species. The key is written so that identification is done in steps. At each step, features of the organism are used to identify which one of two routes through the rest of the key is appropriate to the organism being identified. Dichotomous keys assume a good knowledge of the subject under investigation.

A dichotomous key is best illustrated with an example. Here is the route through a key that was used to identify field maple and sycamore trees: (1) If it is a tree then go to (2). If it is not a tree then go to (5). (2) If leaves are in opposite pairs then go to (3). If leaves are not in opposite pairs then go to (5). (3) If leaves are lobed and arranged like fingers on a hand then go to (4). If leaves are not so arranged then go to (5). (4) If leaves have three lobes then it is Acer campestre (field maple). If leaves have five lobes then it is Acer pseudoplatanus (sycamore). (5) It is not a field maple or sycamore.