Probably the first couplet was written by a Taoist poet of the T'ang Dynasty, but by the10th century, couplets written on scrolls were being used as wall decoration by the educated elite. The Spring couplet, written specially for the New Year, became popular in the 14th century when the first Ming Emperor ordered every household, rich or poor, to have such a decoration on their main door on New Year's day.
Composing couplets became a widely cultivated social accomplishment by the 17th century and was used to test people's education and native wit by setting a line to be matched. Until recently, Chinese children were trained in the writing of couplets by exercises: first single-word opposites, then two-character and seven-character couplets or more. Even today, on funeral scrolls, in temples and shrines, at scenic spots, inside restaurants, publicly and privately, newly composed couplets abound.
A couplet, as its name implies, is made up of two parts, called Head and Tail, with same number of characters in each part, a contrast being provided, either word for word, or phrase for phrase. In painting a couplet, the calligrapher had the choice of either combining the two parts on one scroll or writing each line on a separate scroll and then hanging them symmetrically.
-- The Art of Chinese Calligraphy By Jean Long