A dichotomous key is a series of questions about an organism. The questions are presented in pairs and organized in a way that answering them results in the correct identification of the organism. One example of a dichotomous key is available on the American Museum of Natural History website.
Dichotomous keys, also known as single access keys or sequential, analytical or pathway keys, are tools used by scientists and amateur naturalists to identify organisms of unknown species. They are not intended to be used as classification systems, though the questions may be grouped according to contemporary classification schemes.
When using a dichotomous key, the user is presented with two questions about the organism at hand, and he can only answer one of the questions in the affirmative. Each affirmative answer leads to another pair of questions until the user correctly identifies the organism.
One criticism of dichotomous keys is that the organism being examined may not have all of the qualities represented in the questions the user is asking while trying to identify it. For example, cats typically have fur, but Sphinx cats do not. Therefore, a series of questions designed to identify a cat would incorrectly identify a Sphinx as something else. A similar problem would result if the user did not have complete access to the organism. For example, if a question referred to the shape of an organism's teeth, and the user did not have the opportunity to examine its teeth, the key would not be helpful.