A dichotomous key presents two choices at each step. Start at the first pair, select the statement matching the specimen, and follow the instruction to another pair or a final name. Useful pairs concern the same clear feature, such as “feathers present” and “feathers absent”. Avoid vague terms such as “quite large”.
A key identifies only the items included in it. A specimen fitting neither branch may be outside its scope. To complete a branch, choose a feature that separates the remaining possibilities. Compare the trait at the current step, eliminate mismatches, and continue until one listed identification remains. Do not begin at a familiar final name and work backwards.
Example 1
A key says: feathers → bird; no feathers → step 2. Step 2: six legs → insect; eight legs → spider. Identify a featherless eight-legged specimen.
No feathers leads to step 2; eight legs leads to spider.
Example 2
Two remaining specimens are a feathered bird and a hairy mammal. Choose a paired feature.
“Feathers present / feathers absent” separates them using a clear observable distinction.
Watch out: A key has a defined set of possible identifications. An unlisted organism may not fit its branches.