The name “couplet” originates from the French word meaning riveting or joining two pieces of iron together. It originated in late 16th century with Sir P Sydney’s Arcadia. A couplet is a two line verse, which rhymes and forms a unit alone or as part of a poem. However, not all couplets do rhyme. Chaucer’s Canterbury tales is almost entirely written using rhyming couplets. 
They often form parts of a longer poem such as a ballad. Many of the poems built on quatrain structure contained paired couplets. During 18th and 19th century, couplets often formed part of plays and often livened up a drawn out play. Here’s a famous couplet I am sure one would easily recognise.
“Good night! Good night! Parting is such sweet sorrow
That I shall say good night till it be morrow.”
This couplet comes from Shakespeare’s play Romeo and Juliet. Shakespeare often used rhyming couplets at the end of scenes to signal the ending. Shakespeare also used a couplet to end each stanza in his sonnets. Often whole poems are written in couplet form — two lines of rhyming poetry, followed by two more lines with a different rhyme, and so on. Robert Frost, one of America’s greatest poets, wrote many poems using couplets. Rhyming iambic pentameter was introduced to English by Chaucer in “The Prologue to the legend of good women” in fourteenth century and became popular as a “heroic couplet.” John Milton popularised the octasyllabic form in 17th century in L’allegro and Il’ Penseroso.
There are two forms of couplets recognised – open and closed couplets. In an “open” couplet the first line “runs into” the next line whereas in a “closed” couplet, the sentence the line tends to end with the last phrase or word.
One can often see couplets written on doorways in China during Chinese New Year. 
These couplets, called “chunlian,” are bought a few days before their new year and stuck on the doorway to hope for prosperity for the coming year. Most of the ancient Tamil scriptures were written in couplet form. Thirukkural, dating back to 300 BCE 
according to some people, is a moral code of conduct, somewhat in the lines of Manu smriti in Sanskrit and the Hammurabi’s code from Mesopotamia. Thirukkural is made up of 1330 couplets in ancient Tamil and is considered one of the greatest works of ethics and morality.
Alexander Pope’s ‘Essay on Criticism’ –
‘In Poets as true Genius is but rare,
True Taste as seldom is the Critick’s Share;
Both must alike from Heav’n derive their Light,
These born to Judge, as well as those to Write.’
Alexander Pope
Here is my example of a short couplet. Story of unremitting love is as old as poetry itself. Poets have used each and every form of poetry to eulogise love. Couplets do give a freedom to express one’s feelings a lot quicker and with emotive aspect coming out in every verse.
The Love of my dreams
It’s with love I cherish
My girl without a blemish.
It’s with love I long for
The girl I’d live for.
It’s with passion I care
Neither pain nor loss I care.
It’s with pleasure I endure
My girl who is so pure.
It’s my love, my dream
She’s there in my every dream.
Shankar Kashyap
shankarkashyap 