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Key concept
A dichotomous key is a series of paired statements (couplets) that progressively narrow down the identity of an organism. At each step you choose one of two options based on observable features.
Dichotomous keys are a practical classification tool used to identify organisms. The word "dichotomous" means "dividing into two": at each step, you must choose between exactly two options. Cambridge examiners test both using keys to identify organisms and constructing your own key from given information.
How to Use a Dichotomous Key
Start at statement 1 and read both options. Choose the statement that matches the organism you are identifying. Follow the instruction: either go to the next numbered pair or arrive at an identification.
Example key for four vertebrates:
a) Has feathers → Bird b) Does not have feathers → Go to 2
a) Has scales → Go to 3 b) Does not have scales → Mammal
a) Has fins → Fish b) Does not have fins → Reptile
Notice that each step gives exactly two choices, and every path leads to one identification.
How to Construct a Dichotomous Key
When asked to build a key, follow these rules:
Use observable features only: colour, size, shape, number of legs, presence/absence of structures. Never use habitat or behaviour.
Each couplet must have exactly two contrasting statements: avoid vague terms like "big" or "small" without a clear comparison.
Start with the most obvious distinguishing feature: this separates the largest groups first.
Use positive statements where possible: "has wings" is better than "does not have legs".
Every organism must reach a unique endpoint.
Exam Tips for Key Questions
Examiners commonly ask you to:
Use a key to identify organisms from a diagram or description: work through step by step.
Complete a partially finished key: identify which organisms are missing and add the correct couplet.
Construct a key from scratch: given 4–6 organisms with listed features.
Common trap: Using overlapping features. If two organisms both "have spots", you cannot use "has spots" as a distinguishing feature in a couplet separating them. Pick a feature that clearly divides the group into two.
Try a focused question (4 marks)
Construct a dichotomous key to identify these four organisms: butterfly, spider, crab, centipede. (4 marks)
Show the answer and marking guidance
1a) Has 6 legs → Butterfly; 1b) Has more than 6 legs → Go to 2.
2a) Has 8 legs → Spider; 2b) Has more than 8 legs → Go to 3.
3a) Has 10 legs / two body sections → Crab; 3b) Has many pairs of legs / long segmented body → Centipede.
Each step must use observable features and lead to a unique identification.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Using subjective or vague descriptions like "big" or "looks like" instead of measurable features.
Creating couplets that do not clearly separate organisms: both options match the same organism.
Forgetting that each step must offer exactly two choices, not three or more.