Glossary of Plant Systematics (1st ed.) by Simpson (2006)
- A character with only two character states.
Glossary of "Cladistics (2nd ed.)" by Kitching et al. (1998)
- A character that has only two observed states. Binary characters are generally coded 0 / 1. They can be directed or undirected, polarized or unpolarized, but are intrinsically ordered. Binary characters cannot be unordered. See also multistate character.
- Kitching, I. J., Forey, P. L. Humphries, C. J. & Williams, D. M. 1998. Cladistics, 2nd ed.: the Theory and Practice of Parsimony Analysis. -xii + 228 pp. Oxford University Press, New York.
- Simpson, M. G. 2006. Plant Systematics. -xi + 590 pp. Elsevier Academic Press, Oxford.