A couplet is a pair of successive lines of metre in poetry. A couplet usually consists of two successive lines that rhyme and have the same metre. A couplet may be formal (closed) or run-on (open). In a formal (or closed) couplet, each of the two lines is end-stopped, implying that there is a grammatical pause at the end of a line of verse. In a run-on (or open) couplet, the meaning of the first line continues to the second.
The word "couplet" comes from the French word meaning "two pieces of iron riveted or hinged together." The term "couplet" was first used to describe successive lines of verse in Sir P. Sidney's Arcadia in 1590: "In singing some short coplets, whereto the one halfe beginning, the other halfe should answere." 
While couplets traditionally rhyme, not all do. Poems may use white space to mark out couplets if they do not rhyme. Couplets in iambic pentameter are called heroic couplets. John Dryden in the 16-17th century and Alexander Pope in the 18th century were both well known for their writing in heroic couplets. The Poetic epigram is also in the couplet form. Couplets can also appear as part of more complex rhyme schemes, such as sonnets.
Rhyming couplets are one of the simplest rhyme schemes in poetry. Because the rhyme comes so quickly, it tends to call attention to itself. Good rhyming couplets tend to "explode" as both the rhyme and the idea come to a quick close in two lines. Here are some examples of rhyming couplets where the sense as well as the sound "rhymes":
On the other hand, because rhyming couplets have such a predictable rhyme scheme, they can feel artificial and plodding. Here is a Pope parody of the predictable rhymes of his era:
Rhyming couplets are often used in Early Modern English poetry, as seen in Chaucer's The Canterbury Tales. This work of literature is written almost entirely in rhyming couplets. Similarly, Shakespearean sonnets often employ rhyming couplets at the end to emphasize the theme. Take one of Shakespeare's most famous sonnets, Sonnet 18, for example (the rhyming couplet is shown in italics):
Chinese couplets or "contrapuntal couplets" may be seen on doorways in Chinese communities worldwide. Couplets displayed as part of the Chinese New Year festival, on the first morning of the New Year, are called chunlian. These are usually purchased at a market a few days before and glued to the doorframe. The text of the couplets is often traditional and contains hopes for prosperity. Other chunlian reflect more recent concerns. For example, the CCTV New Year's Gala usually promotes couplets reflecting current political themes in mainland China.
Some Chinese couplets may consist of two lines of four characters each. Couplets are read from top to bottom where the first pline starts from the right. But is also a 6 word diagraph with 19 lines
Deep summer, and time passes.Sorrow wastes
To a new sorrow.While Time heals time hastes