A couplet is one of the simplest poetic structures. One couplet contains two lines that usually rhyme. A couplet can belong to a larger stanza or constitute a stanza on its own. The lines in a couplet can take any length; they do not even have to share the same number of syllables. The only distinguishable features of a couplet are two lines and a possible rhyme scheme. Normally a couplet within a larger stanza rhymes; otherwise, differentiating it from the rest of the stanza is difficult.
The last word in the lines of a rhyming couplet rhyme. Rhymes can take many forms. They can rhyme exactly, as "perfect" rhymes do, such as "lake" and "rake," or be "imperfect" rhymes, words that almost rhyme, such as "lap" and "shape." Assonant rhymes share similar vowel sounds but have different consonants, such as "won" and "come." An eight-line couplet poem contains four couplets and therefore four rhymes. All of these rhymes can be the same, such as "aa aa aa aa," or they can take a different rhyme every time, such as "aa bb cc dd" or any other variation of couplet rhyming a writer desires, even "aa bb aa bb."
Sonnets, one of the more well-known poetic forms, typically end with a rhyming couplet: "So long as men can breathe or eyes can see, / So long lives this, and this gives life to thee," ends Shakespeare's famous "Shall I Compare Thee to a Summer's Day?" sonnet. William Carlos Williams poem "XXII" is an eight-line couplet, although it does not rhyme:
"so much depends
upon
a red wheel
barrow
glazed with rain
water
beside the white
chickens"
As evident in Williams' brief poem, couplets lend themselves to minimalist subjects. An eight-line couplet poem is fairly short. Writers desiring to capture a brief event or emotion or a series of images or sensations might find an eight-line couplet poem helpful. Poets who struggle with rhyming can consult a rhyming dictionary for help.