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Wu (simplified Chinese: ??; traditional Chinese: ??; pinyin: Wú y?, Suzhou Wu: [??? ?y??]) is a grouping of linguistically similar and historically related varieties of spoken Chinese also known as a dialect family. Chinese varieties classified as Wu are primarily spoken in Zhejiang province, the municipality of Shanghai, and southern Jiangsu province.
Major Wu dialects include those of Shanghai, Suzhou, Ningbo, Wenzhou, Hangzhou, Shaoxing, Jinhua, and Yongkang. Wu speakers such as Chiang Kai-shek, Lu Xun, and Cai Yuanpei occupy places of great significance in modern Chinese culture. Wu can also be found in the language of Yueju opera which is second in national popularity only to Peking Opera as well as in the performances of the nationally popular entertainer Zhou Libo. Wu is also spoken in a large number of diaspora communities, with significant centers of immigration originating from Qingtian and Wenzhou.
Suzhou has traditionally been the linguistic center of the Wu languages and was likely the first place a distinct variety of Chinese known as Wu developed. Suzhou Wu is widely considered to be the most linguistically representative of the family. It was mostly the basis of the Wu pidgin that developed in Shanghai leading to the formation of modern Shanghainese which as a center of economic power and possessing the largest population of Wu speakers has attracted the most attention. Because of the influence of Shanghainese, Wu is often inaccurately referred to in English as simply "Shanghainese" when introducing the dialect family to non-specialists. Wu is the more accurate terminology for the greater grouping that the Shanghai dialect is part of; other less precise terms include "Jiangnan speech" (???), "Jiangzhe (Jiangsu-Zhejiang) speech" (???), and less commonly "Wuyue speech" (???).
This dialect family (and especially Southern Wu) is well-known among linguists and sinologists as being one of the most internally diverse among the spoken Chinese language families with very little mutual intelligibility among varieties within the family. Among speakers of other Chinese varieties, Wu is often subjectively judged to be soft, light, and flowing. There is an idiom in Chinese that specifically describes these qualities of Wu speech: Wú nóng ru?n y? (????), which literally means "the tender speech of Wu." On the other hand, some Wu varieties like Wenzhounese have gained notoriety for their incomprehensibility to both Wu and non-Wu speakers alike, so much so that Wenzhounese was used during the Second World War to avoid Japanese interception.
Along with Germanic languages, Wu dialects have the largest vowel quality inventories in the world. The Dôn?äc or Jinhui dialect spoken in Shanghai's Fengxian District has 20 vowel qualities, the most among all world languages.
Wu dialects are typified linguistically as having preserved the voiced initials of Middle Chinese, having a majority of Middle Chinese tones undergo a register split, and preserving a checked tone typically terminating in a glottal stop, although some dialects maintain the tone without the stop and certain dialects of Southern Wu have undergone or are starting to undergo a process of devoicing. The historical relations which determine Wu classification primarily consist in two main factors: firstly, geography, both in terms of physical geography and distance south or away from Mandarin, that is Wu dialects are part of a Wu-Min dialect continuum from southern Jiangsu to southern Fujian and Chaozhou. The second factor is the drawing of historical administrative boundaries which in addition to physical barriers limit mobility and in the majority of cases more or less determine the boundary of a Wu dialect.
Wu Chinese along with Min are also of great significance to historical linguists due their retention of many ancient features. These two families have proven pivotal in determining the phonetic history of the Chinese language.
More pressing concerns of the present are those of dialect preservation. Many within and without the country fear that the increased usage of Mandarin may eventually supplant the languages altogether which have no written form, legal protection, or official status and are officially barred from use in public discourse. However, many analysts believe that a stable state of diglossia will endure for at least several generations if not indefinitely.

http://en.wikipedia.org/wiki/Wu_Chinese