# Explain how the use of a dichotomous key allows an organism to be identified and assigned to a known species.

> IB DP Biology Standard Level (2023 syllabus) — A3.1 Diversity of organisms · Explain · 4 marks

## Mark scheme (4 marks)

1. A dichotomous key presents a series of paired statements (couplets) with contrasting observable characteristics.
2. At each couplet the user selects the statement that matches the observable characteristics of the organism, directing them to the next couplet.
3. By following through successive couplets, the number of possible species is progressively narrowed down.
4. The process ends when a species name (binomial) is reached, thereby assigning the organism to a known species.

## Key terms

- [dichotomous key](https://www.gradenine.co.uk/glossary/dichotomous-key)
- [species](https://www.gradenine.co.uk/glossary/species)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-use-of-a-777b07d8) · Published by Druglandscape Ltd.