A dichotomous key is a series of questions with only two alternatives. There there are only two options at each step. The purpose is to make an identification or to reach a decision. The system has long been used for identifying animals and plants. However, in principle, it can be used for many other tasks, such as fault-finding in complex machinery.
This key first differentiates between oaks with entire leaves with normally smooth margins (live oaks, Willow oak, Shingle oak), and other oaks with lobed or toothed leaves. The following steps created smaller and smaller groups (e. g., red oak, white oak), until the species has been keyed out.