Field guides help you name an organism you do not know. Many use a dichotomous key: "dichotomous" means "cut in two." It turns classifying into a path of either-or choices.
Key idea
A dichotomous key is a list of numbered steps. Each step gives two choices about a trait you can observe. Pick the one that fits, and it sends you to another step or gives the organism's name. Always start at step 1.
Why it works
Each choice splits the remaining possibilities into two groups, so every step rules out a share of them. The traits are things anyone can check, like legs or wings, so two people using the same key on the same specimen reach the same name.
Worked examples
Worked example: Walk the key
A specimen has six legs and wings covered with colored scales. Use the key above.
- Step 1: it has legs, so go to step 2.
- Step 2: six legs, so go to step 3.
- Step 3: it has wings, so go to step 5.
- Step 5: colored scales on its wings.
Answer: A butterfly.
Worked example: No legs
A specimen has no legs and a shell. What is it?
- Step 1: no legs, so go to step 4.
- Step 4: it has a shell.
Answer: A snail.
Common mistake
Jumping ahead or answering from what you think the organism is ("It looks like a bee, so it is a bee"). Read both choices at every step, in order, and go only where the key sends you. Check by writing down the steps you used: 1 → 2 → 3 → 5.