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Dichotomous keys are the most common keys encountered. They may be laid out in various ways, but usually form a series of numbered questions arranged in “couplets” as shown below:

1. Bark on trunk smooth .......................... 2
  Bark on trunk rough ............................ 3
     
2. Bark mostly white ............................... 4
  Bark other colours .............................. 7

Each time a question is answered, the user is directed to the number of a new question-couplet. This continues until, instead of a number, the name of the species (or other taxon) is given. This type of key is called a “dichotomous” key because the meaning of the word is "two branching", although in practice dichotomous keys often have questions with more than two choices. The structure of the key is such that each question is actually like a tree branch that has smaller branches proceeding from it, as demonstrated below for keying out four "egg-laying animals".

Dichotomous Key Structure

    
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