||This article needs additional citations for verification. (December 2006)|
Chengyu (simplified Chinese: 成语; traditional Chinese: 成語, pinyin: chéngyǔ, lit. "set phrases") are a type of traditional Chinese idiomatic expressions, most of which consist of four characters. Chengyu were widely used in Classical Chinese and are still common in vernacular Chinese writing and in the spoken language today. According to the most stringent definition, there are about 5,000 chengyu in the Chinese language, though some dictionaries list over 20,000.
They are often referred to as Chinese idioms or four-character idioms; however, they are not the only idioms in Chinese.
Chengyu are mostly derived from ancient literature. The meaning of a chengyu usually surpasses the sum of the meanings carried by the four characters, as chengyu are often intimately linked with the myth, story or historical fact from which they were derived. As such, chengyu do not follow the usual grammatical structure and syntax of the modern Chinese spoken language, and are instead highly compact and synthetic.
Chengyu in isolation are often unintelligible without explanation, and when students in China learn chengyu in school as part of the classical curriculum, they also need to study the context from which the chengyu was born. Often the four characters reflect the moral behind the story rather than the story itself. For example, the phrase "break the woks, sink the boats" (破釜沉舟, pò fǔ chén zhōu) is based on a historical account where the general Xiang Yu ordered his troops to destroy all cooking utensils and boats after crossing a river into the enemy's territory. He won the battle because of this "no-retreat" strategy. Similar phrases are known in the West, such as "burning bridges" or "Crossing the Rubicon". This particular idiom cannot be used in a losing scenario because the story behind it does not describe a failure.
Another example is "瓜田李下" ( guātián lǐxià, lit. "melon field, beneath the plums"). It is an idiom that has a deeper meaning that implies suspicious situations. It is derived from an excerpt from a Han era poem (樂府詩《君子行》, Yuèfǔ Shī "Jūnzǐ Xíng"). The poem includes the lines, "Don't adjust your shoes in a melon field and don't tidy your hat under the plum trees" (瓜田不納履，李下不整冠, gūatián bù nà lǚ, lǐ xià bù zhěng guān), admonishing the reader to avoid situations where however innocent he might be suspected of doing wrong. The literal meaning of the idiom is impossible to understand without the background knowledge of the origin of the phrase. However, some idioms such as "wind from an empty cave" (空穴來風, kōng xué lái fēng, viz. "hot air") and "bare-faced facing the emperor" (素面朝天, sù miàn cháo tiān, viz. "without makeup") are so widely misunderstood that their literal meanings have overtaken their original ones. "wind from an empty cave" is now currently mistakenly used to describe rumors without source when the actual meaning is the opposite. It is used to describe rumors with actual, solid sources or reasons. "Bare-faced facing the emperor" is now misused to describe beauties. (Beauties who don't require make-up when entering court) Its real meaning is "to be confident in one's true look".
However, that is not to say that all chengyu are born of an often-told fable. Indeed, chengyu which are free of metaphorical nuances pervade amidst the otherwise contextually driven aspect of written vernacular Chinese. An example of this is "speaking, yet without trust" (言而无信, yán ér wú xìn), referring to an individual who cannot be trusted despite what he says, an essentially deceitful person. The idiom itself is not derived from a specific occurrence from which a moral may be explicitly drawn; instead, it is succinct in its original meaning and would likely be intelligible to an individual learned in formal written Chinese. Its archaic nature is only betrayed by the now-unusual use of the character yán (言) as a verb.
Many Chinese idioms have their English equivalents. For example, 冰山一角 and "the tip of the iceberg" share both their literal and idiomatic meanings, while 言不由衷 and "to speak with one's tongue in one's cheek" share idiomatic meanings.
Chinese idioms can also serve as a guide through Chinese culture. Chengyu teach about motifs that were previously common in Chinese literature and culture. For example, idioms with nature motifs – e.g., mountains (山), water (水), and the moon (月) – are numerous. Works considered masterpieces of Chinese literature – such as the Four Great Classical Novels – serve as the source for many idioms, which in turn condense and retell the story.
Chinese examples 
The following three examples show that the meaning of the idiom can be totally different by only changing one character.
一 日 千 秋: "One day, a thousand autumns."
- Meaning: implies rapid changes; one day equals a thousand years
一 日 千 里: "One day, a thousand miles."
- Meaning: implies rapid progress; traveling a thousand miles in a day
一 日 三 秋: "One day, three autumns."
- Meaning: greatly missing someone; one day feels as long as three years
Other examples in Chinese:
- 三人成虎 (Three men make a tiger)
Japanese examples 
Yojijukugo is the similar format in Japanese. The term yojijukugo (四字熟語, "four character idiom") is autological. Many of these idioms were adopted from their Chinese counterparts and have the same or similar meaning as in Chinese. The term koji seigo (故事成語, "historical idiom") refers to an idiom that comes from a specific text as the source. As such, the overwhelming majority of koji seigo comes from accounts of history written in classical Chinese. Although a great many of the Japanese four-character idioms are derived from the Chinese, many others are purely Japanese in origin. Some examples:
- 起承転結 ki, shō, ten, ketsu ("Start, Continue, Change, Conclusion" – a traditional structure for composing Tang poetry.)
See also 
Dictionaries of Mandarin Chinese Idioms 
- Herbert Allen Giles (1873). A dictionary of colloquial idioms in the Mandarin dialect. SHANGHAI: A.H. de Carvalho. p. 65. Retrieved 10th of February, 2012.(Harvard University)(Digitized Jul 22, 2005)
- Herbert Allen Giles (1873). A dictionary of colloquial idioms in the Mandarin dialect. SHANGHAI: A.H. De Carvalho. p. 65. Retrieved 10th of February, 2012.(Harvard University)(Digitized Mar 4, 2009)