A dichotomous key is a reference tool used mainly in biology. It allows the user to determine what an organism is by answering numerous questions that have only two possible answers. Ultimately, the answers will point to a specific organism or some higher taxonomic level. Dichotomous keys can be very broad or extremely specific. To create a key, you need a working knowledge of the differences between groups of organisms. The more specific the key, the more knowledge you need.
Determine the level of specificity your key will need. For instance, your key can simply identify different phyla of animals or it can try to determine exactly what type of bacteria a user is observing.
Develop questions that will differentiate between organisms. Broad questions can be along the lines of "Is it eukaryotic or prokaryotic?" More specific questions would be along the lines of "Do the bacteria grow in the presence of 6.5-percent sodium chloride broth?" Each question should have only two possible responses.
Keep developing questions that will differentiate between more and more similar organisms until you reach the level of specificity you desire.
Write the first question on your key at the top of the page in the center, and draw two lines branching out from beneath it. Each line will point to one possible answer. Write the answers at the end of each line.
Continue branching your questions in the same fashion. If at any time you can positively identify a species or group, write the species or group name below the answer to the last question. Example dichotomous keys can be found by following the reference links.