couplet in taxanomic key means

Taxonomic keys are written characteristic information that helps a person to identify an unknown species of living organism. The taxonomic key provides a certain structure on the basis of which the user can sort out the taxonomic position of the unknown species.

These keys are written in series of couplets. Couplet contains pair of contrasting characters which means two opposite description of an organism. The users sort out the description that best fits the organism.