In biology, a classification key is a means of categorizing living organisms by identifying and sorting them according to common characteristics. Also referred to as a dichotomous key, single-access key or pathway key, the process enables organism classification by answering a series of hierarchical and specific identification questions by choosing one of two possible answers. The answer chosen leads to the next two-answer-choice question, which then continues in a sequential order that further narrows down the organism's characteristics and ultimately leads to its final categorization.
An example of a simple classification-key exercise that might be used to illustrate the concept to students would begin with a starting question such as, "Is it an animal or a plant?" If the answer is "animal," the next set of two questions could be, "Does it walk or fly?" If the answer to the first question was "plant" instead of "animal," the next question might be, "Does it have a trunk or a stem?"
A classification key that is used to organize living things is also called a biological key. The first use of the process is credited to the fourth-century B.C. scientist and philosopher, Aristotle. Using the dichotomous, two-branch questioning approach, Aristotle began by first asking if the organism did or did not possess red blood. The answer "no red blood" led to the next question, "Does it have a hard body or a soft body?" The answer "soft body" led to the next question, "Does it have a shell or no shell?" Continuing down either branch of the original "red blood or no red blood" question enabled Aristotle to classify various organisms such as mammals, birds, lizards, fish, shellfish and insects. By doing so, Aristotle developed one the first systems of scientific classification.