Because of characteristic of Chinese characters, couplets are Chinese characteristic literary patterns.
Why do couplets have sense of beauty? Besides its content or theme and conception, the importance is that the making of couplets uses plentiful and wonderful rhetoric technique, which is a skill of making language more accurate, more distinctive, more vivid and more lively. Almost every rhetoric technique can be used in couplets -- Chinese characters have one of the characteristics: A word has many meanings. Using rhetoric technique can lead to a phrase with a double meaning; Chinese characters also have a kind of characteristic: A word has a few speech sound. Using rhetoric technique can lead to a sound with a double meaning; Chinese characters have a characteristic of pictophonetic characters, with one element indicating meaning and the other sound. Using rhetoric technique can form "Xi Zi Lian" -- a kind of couplets that analyze words-: Owing to the use of rhetoric technique, we can appreciate all kinds of ingeniously constructed and exceIlent artistic conception couplets. Couplets more or less include all rhetoric skiIIs (Rhetoric Case). All the rhetoric skills in it must also accord with the demands of antithesis and must use them in antithesis. The rhetoric use of the first line and the second line of couplets must have the same numbers of words, different characters, the same part of speech, relevant sentence patterns, interrelated content and harmonious tonal patterns. So couplets are rhetoric among rhetoric. This is the first characteristic of rhetoric art of couplet.
Due to the first characteristic, rhetoric can form some unique patterns when it's used in couplets. First, so long as one couplet refers to the came thing, and the first and the second sentences are related, the antithesis demands of this pair of couplets may be relaxed restrictions properly, and break that antithesis pays special attention to the opposite demands of numeral-classifier compound, words of colors, and noun of locality. Second, because it is limited by think-in-itself and the words of metaphor and neither of them appear, "loaned-metaphor" in couplet can only be used with the first line and the second line of a couplet at the same time, otherwise it can't be antithesis. Third, the rhetoric uses of a phrase with a double meaning、a sound with a double meaning、"Xi Zi" -- analysis words and "Qian Zi" -- inlay words etc. in couplet often appear one meaning in words and express another meaning in deeds; one light and one shade, light is secondary, shade is primary, especially the meaning in words must be clear and suitable, readable and fluent. You can't draw a forced analogy in order to achieve the goal of pun, analysis words and inlay words etc., or you'll try to be clever only to end up with a blunder. Fourth, besides put up or hung, couplets are more often used in communications, social intercourse. It belongs to an oral creation. An oral creation doesn't deviate from concrete time、places、characters and reasons etc.. that is concrete language environments of couplet creation. If the environment changes or an object changes, some rhetoric skills can't be handled in couplets. Not knowing concrete language environment, readers can't also understand some marvelous rhetoric technique in couplets, and so on and so forth. This is the second characteristic of couplet rhetoric art.
Because of couplet special functions and language environments while couplets are made, some rhetoric technique can seldom or hardly be used in other literature patterns, but they are used more often, even only in couplets, such as: "Xi Zi'、"Qian Zi"、"Yong Shu"、 "Ji Ju"、"Bie Jie"、"Jia Cheng"、"Fei Bai"、"Xu Zhuan" and so on. This is the third characteristic of couplet rhetoric art.