Shanghainese is a dialect of Wu spoken by about 15 million people in Shanghai, also known as the Shanghai language. The term Wu comes from the historic Kingdom of Wu first united by Wu Taibo with its capital is located just 80 km from today’s Shanghai during the Autumn and Spring period 2500 years ago, founded again during the Three Kingdoms period 1700 years ago, and again during the Five Dynasties and Ten States period as the Kingdom of Wuyue. Today's Wu-speaking region covers half of ancient Wu in Jiangsu province and most of the archrival Yue Kingdom in Zhejiang province. There are also many Shanghainese speaking people in Hong Kong.
Shanghainese has no standard written characters, as it rarely appears in writing. Though the language is the everyday spoken language of Shanghai, it isn't used in education and it is only heard in Shanghai and nearby regions. There are international students learn Shanghainese in order to develop business relationship with local people in Shanghai.
The Common Chinese Language movement was started by Shanghai intellectuals and writers during the early 20th century to create a common vernacular medium for national communication. Mandarin was selected as the base, due to the large number of Mandarin speakers in China and its relative simplicity. The bulk of vernacular Mandarin Chinese literature was written not by native Mandarin speakers but by native Wu and Shanghainese speakers.
As result, a lot of today's Mandarin Chinese vocabulary comes from Wu Chinese via these literary works. The words and usages have become so well adapted into Standard Mandarin that most speakers assume they are indigenous to Mandarin rather than being cognates of Shanghainese.
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