How are couplets used in other works of literature besides sonnets, like Shakespeare's sonnets?

Couplets--which are two lines of poetry that generally
rhyme--are found in the poetry of many peoples, for example, in the
Tamil poetry of India and the
duilian poetry of China. In English
poetry, rhyming couplets were a feature of the first English language poetry written by
Geoffrey Chaucer: the famous The Canterbury
Tales are written in rhyming couplets as are other
Chaucerian masterpieces, like The Book of the
Duchess:

readability="17">

I have gret wonder, be this
lighte,
How that I live, for day ne nighte
I may nat slepe wel nigh
noght,
I have so many an ydel thoght

REWRITTEN IN
CONTEMPORARY ENGLISH
I have great wonder, by this light,
How that I
live, for day nor night
I may not sleep very near to naught,
I have
so many an idle
thought.

Couplets are used in
a number of poetic forms (poetic form: the specifications that govern different types of
poetry, which may include rules about rhyme, meter, structure etc). A very popular form
is the heroic couplet, which has five rhythmic divisions,
called pentameter. To illustrate, Chaucer's lines above
have four feet and so are in tetrameter: therefore these are not heroic
couplets.

Another example is that couplets are also a feature of
elegies. In English poetry, these are laments for a
deceased person that mimic Roman elegies that have a complex structure and are written
in rhyming couplets. John Milton wrote elegiac laments like
Lycidas, which has a complex rhyme scheme that employs couplets
(along with other schemes) as in this sample with three rhyming couplets
(spring / string; excuse /  Muse; Urn /
turn):

readability="13">

That from beneath the seat of Jove doth
spring,
Begin, and somwhat loudly sweep the string.
Hence with
denial vain, and coy excuse,
So may som gentle Muse
With lucky words
favour my destin'd Urn, [ 20 ]
And as he passes turn,
....