"A Dictionary of Ecology, Evolution and Systematics (2nd ed.)" by Lincoln et al. (1998)
- A dichotomous key in which contrasting parts of a couplet are numbered and presented together, without intervening couplets, although the brackets joining each couplet (from which the key derives its name) are now omitted; bracket key; parallel key; cf. dichotomous key, indented key.
- Lincoln, R., Boxshall, G. & Clark, P. 1998. A Dictionary of Ecology, Evolution and Systematics. ix + 361 pp. Cambridge University Press, Cambridge.