The big idea: A dichotomous key is a tool that identifies an unknown organism by asking a series of either/or questions about its observable features.
Each step gives you two choices. You pick the one that matches your organism, and it sends you to the next step — or finally names the organism.
'Di-chotomous' literally means 'split into two' — every step branches two ways.
How you read a key
- Start at Step 1 — always begin at the top
- Read both choices in the pair before deciding
- Pick the choice that matches the organism in front of you
- Follow the instruction: go to another step, or read off the name
- Keep going until the key gives you an organism's name
Two key terms: Observable feature = something you can see or measure (legs, shell, scales, wings) — not behaviour and not where it lives.
Either/or pair = the two opposite choices offered at each step.
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A good dichotomous key uses features that are clearly opposite, so at each step the organism fits one choice and not the other.
The choices must be about features you can actually observe — for example 'has six legs' versus 'has eight legs', not 'lives in a garden' versus 'lives in a pond' (where it lives is not a body feature).
- Dichotomous key
- An identification tool made of paired either/or statements, each leading to another step or to the name of an organism.
- Observable feature
- A characteristic that can be seen or measured directly, such as number of legs, presence of a shell, or type of skin.
- Couplet (step)
- One numbered pair of opposite statements in the key — you choose the one that matches your organism.
- Keys out
- The point at which the key finally identifies (names) the organism.
A GOOD key choice
- 'Has a shell' vs 'has no shell' — opposite and clear
- Based on a feature you can see
- Every organism fits exactly one side
A POOR key choice
- 'Big' vs 'small' — vague, depends on the specimen
- 'Lives in water' — a habitat, not a body feature
- Choices that overlap, so an organism fits both
The golden rule: At every step, follow only the choice that matches your organism, then do exactly what that line tells you — go to the next numbered step or read off the name.
Never skip a step, and never pick a choice just because it sounds likely.
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How this is tested: On Paper 1A (multiple choice) you are usually given a small dichotomous key and an organism (often shown as a picture or a list of features). You must follow the key and choose the letter or name the organism keys out.
The marks are won by working through the key step by step — not by guessing from the name. Read both choices at each step before you move on.
Use this key for the worked question below. Each numbered step gives two choices; follow the one that matches the animal.
| Step | Choice | Where to go |
|---|---|---|
| 1 | Body has a hard outer skeleton (exoskeleton) and jointed legs | → go to Step 2 |
| 1 | Body is soft, with no jointed legs | → go to Step 3 |
| 2 | Three pairs of legs (six legs) | → Insect |
| 2 | Four pairs of legs (eight legs) | → Spider |
| 3 | Has a coiled shell | → Snail |
| 3 | Has no shell | → Slug |
IB-style question — follow the key to identify the animal
A small garden animal has a soft body with no jointed legs, and it carries a coiled shell on its back. Use the key above to identify it. [1]
How to work through the key
- Start at Step 1. The animal has a soft body with no jointed legs, so take the second choice → go to Step 3 (not Step 2, which is for hard, jointed-leg bodies).
- At Step 3, check the shell. The animal has a coiled shell, so take the first choice → this keys out as Snail.
- Answer the question: following the key (soft body → Step 3 → has a shell), the animal is identified as a snail.
Final answer
Soft body with no jointed legs → Step 3; has a coiled shell → Snail. The animal is a snail.
The same key drawn as a branching tree. The highlighted route shows the answer: soft body → Step 3 → has a shell → Snail.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Check your route: Your path through the key should read: Step 1 (no jointed legs) → Step 3 (has a shell) → Snail. If you ended anywhere else, you took a choice that did not match the animal — go back and re-read both options at each step.