Glossary of "Plant Biology (1st ed.)" by Graham et al. (2003)
- A tool used for organism identification, whereby the user makes a series of choices between two alternative character descriptions.
Glossary of Plant Systematics (1st ed.) by Simpson (2006)
- A key utilizing series of two contrasting statements, each statement a lead, the pair of leads a couplet.
"A Dictionary of Ecology, Evolution and Systematics (2nd ed.)" by Lincoln et al. (1998)
- An identification key constructed as a sequence of alternative choices, each pair forming a character couplet; diagnostic key; sequential key; cf. bracketed key, indented key.
- Graham, L. E., Graham, J. M. & Wilcox, L. W. 2003. Plant Biology. -xxxv + 497 pp. Pearson Education, Inc., Upper Saddle River.
- Lincoln, R., Boxshall, G. & Clark, P. 1998. A Dictionary of Ecology, Evolution and Systematics. ix + 361 pp. Cambridge University Press, Cambridge.
- Simpson, M. G. 2006. Plant Systematics. -xi + 590 pp. Elsevier Academic Press, Oxford.