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Conventional dichotomous keys

Conventional dichotomous keys consist of a series of paired choices called couplets. Each couplet presents the user with two alternative and exclusive sets of characters that distinguish one group of organisms from the other. These alternatives are essentially questions about the characteristics of the organism which is being identified (i.e. named). By choosing one alternative the user is led to the next couplet where the process is repeated. By progressively working through the key in this way the number of possible answers is narrowed, until at the end only a single taxon (e.g. a species) is left. The couplets in a conventional dichotomous key are pre-determined by the person who constructed the key and these keys that are normally produced as printed hardcopy.

Example of a dichotomous key

1 Flowers arranged in globular heads


2 Branchlets winged


3 Flowers cream coloured

Acacia alata


3: Flowers golden

Acacia stenophylla


2: Branchlets not winged


5 Phyllodes terete (i.e. round in cross-section)

Acacia assimilis


5: Phyllodes flat

Acacia saligna

1: Flowers arranged in cylindrical spikes

Acacia acuminata

Because of their linear structure, conventional dichotomous keys suffer from a serious problem, the ‘unanswerable couplet problem’; that is, if any question in the sequence cannot be answered then further progress through the key is blocked. For example, using the above key consider trying to identify a plant specimen which has no flowers! Furthermore, dichotomous keys can be hard to use, especially when the number of species included is large. Here the key-maker often has to resort to ‘cryptic’ characters that may be difficult to interpret, or to combinations of characters, in order to try and guide the user to the correct answer. In these cases a more effective way of naming species is to use an interactive multi-access key if one exists for the group under consideration.

Despite their shortcomings conventional dichotomous keys are useful (especially when the numbers of species involved is not too large) and a many have been produced for various groups of Acacia.

Page last updated: Thursday 22 June 2023