• the heroic couplet, which is always in iambic pentameter, gets its name from the form used in various 17^th and 18^th century translations of heroic epics
• closed couplets end definitively, usually with a period or some such signifier, while open couplets can flow with the same idea from one to the next
• triplets add a third rhyming line as a variation (as opposed to the tercet, which is three lines to begin with)
• the alexandrine is another variation, generally used as a last line (and often as the third of a triplet); it has twelve syllables, and emphasizes the end of the poem, and usually offers some sort of summary or punch line as well

Coup"let (-l?t), n. [F. couplet, dim. of couple. See Couple, n. ]

Two taken together; a pair or couple; especially two lines of verse that rhyme with each other.

A sudden couplet rushes on your mind. Crabbe.

© Webster 1913.