couplet

noun5

1. •
   A set of two things.
2. •
   A pair of lines, typically with rhyming end words.
3. •
   A pair of one-way streets which carry opposing directions of traffic through gridded urban areas.

   • 5th Street is one-way west only and 6th Street is one-way east only. Together, they form a couplet in Downtown Los Angeles.
4. •
   A pair of two mutually exclusive choices in a dichotomous key.

   • The dichotomous keys are constructed so that each couplet presents a set of alternative choices. — 2001
5. •
   Ellipsis of antithetical couplet, Chinese couplet, or duilian couplet (“duilian”).