A dichotomous key uses paired, opposite statements about visible features to reach a name. Every organism must end in exactly one place.
This lesson is part of biodiversity in the SPM Biology subject guide. Place organisms first with classification hierarchies.
How do you build a key in steps?
- List the organisms and one visible feature for each.
- Pick a feature that splits the list into two sets.
- Write the pair of opposite statements.
- Repeat for each set until every organism stands alone.
Worked example: a key for four animals
Original set: rhinoceros hornbill, Malayan tiger, green tree frog, king cobra.
| Organism | Visible features |
|---|---|
| Hornbill | Feathers, beak |
| Tiger | Fur, four legs |
| Tree frog | Moist skin, no scales |
| Cobra | Dry scales, no limbs |
Step 1: feathers split the hornbill from the rest. Step 2: fur splits the tiger from the two left. Step 3: moist skin splits the frog from the cobra.
The key:
- (a) Body covered with feathers: rhinoceros hornbill. (b) No feathers: go to 2.
- (a) Body covered with fur: Malayan tiger. (b) No fur: go to 3.
- (a) Skin moist, without scales: green tree frog. (b) Skin dry, with scales: king cobra.
Test each organism from the start. The cobra passes 1(b), 2(b) and 3(b), and arrives at its own name.
A common mistake: a pair that is not truly opposite
A learner writes: 1(a) Large animal, 1(b) Small animal. This fails because “large” depends on opinion: a tiger is large beside a hornbill but small beside a whale.
| Pair | Status |
|---|---|
| Large / small | Weak: relative, not observable |
| Can fly / cannot fly | Weak: bats and hornbills both fly yet belong to different groups, and some birds cannot fly |
| Has feathers / no feathers | Strong: one visible, yes or no feature |
Choose features you could check in a photo.
Reading a key someone else wrote
Start at statement 1. Read both halves, choose the one that matches your organism, and follow the instruction. Keep going until you reach a name, not a number.
Check yourself
Add a fifth animal, a mudskipper with moist skin and fins, to the key above. Which pair needs a new split, and what feature would you use?
Answer
The mudskipper has no feathers and no fur, so it passes pairs 1 and 2 and reaches pair 3. Its moist skin matches the frog, so pair 3 needs a new split.
Change pair 3 to: (a) Skin dry, with scales: king cobra. (b) Skin moist: go to 4.
Add pair 4: (a) Fins present: mudskipper. (b) Four limbs, no fins: green tree frog.
Trace all five animals from pair 1. Each one should reach its own name, and no two should share a name.
What to study next
Move on to comparing major organism groups, where the features that split groups are explained. Practise data-style questions with the biological data interpretation trainer.
If you want a teacher to test your key with you, see online one-to-one Biology tuition.