A dichotomous key is a type of single-access key that offers only two choices at one time. These keys are commonly used in biology in order to identify an unknown animal or plant. In order to use these keys, the user determines which of the two different choices is correct and then follows the key until a final choice eventually leads to the name of the organism. Though a dichotomous key can be made for any group of objects, they were first created for use in biology and are most commonly seen in this field.
First used a few hundred years ago, dichotomous keys are made specifically to identify various lifeforms within a group. These keys work best when used to identify species that are closely related but that have distinctive features or behaviors that distinguish them from the other species in the group, such as finches of the Galapagos Islands. Though it is possible to make a dichotomous key for a large group, such as reptiles, the vast number of species in this group makes it impractical for this type of use.
In a well-organized dichotomous key, the first choice will split the group into two groups that have roughly the same number of species each. Moving through the key will narrow the group down into smaller and smaller groups until certain choices begin to identify individual species. They may be arranged in a linear fashion, which works well for small groups without many choices, or into branches.
It is possible to adapt a dichotomous key for use outside the biological sciences as well. Any group of different objects that require classification, such as stones or chemicals, can be identified using this type of key. Keys can also be created specifically for certain groups of things so that extraneous choices that would not be found in a certain setting can be eliminated.
The trouble with using a dichotomous key is that in some cases, neither of the choices will be true. The keys are based on information that is correct for most members of a species, or that is true at most times, but may not be observed in an individual specimen. For example, a choice that identifies the number of spots on a gecko's tail as 5 or 7 may be impossible to answer if the gecko has lost its tail. In this case, the creature may be impossible to correctly identify or it may be necessary to follow multiple threads through the key, which can be confusing.