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Standard English is a general term for a form of written and spoken English that is considered the model for educated people by native English speakers. There are no set rules or vocabulary for "Standard English" because, unlike languages such as French or Dutch, English does not have a governing body (see Académie française, Dutch Language Union) to establish usage. As a result, the concept of "standard English" tends to be fluid. Various regional and national "standards" exist.
The issue is particularly complicated because English has become the most widely used language in the world, and therefore it is the language most subject to alteration by non-native speakers.
The two most common recognised varieties of English are Standard English and American English. Commonwealth English (based on British English) is a term used chiefly by Americans to refer to non-American English. However, the various historical migrations of English-speaking populations, colonisation and attendant effects such as creolization, the use of English as a lingua franca or trade language and its widespread use internationally has given rise to many local varieties of English.
Standard English is a language not an accent: that is, Standard English refers to a set of words, grammar, and linguistic sounds, and not to a particular pronunciation. One example would be the (correct) statement, "I feel well." as opposed to the oft-heard but incorrect "I feel good".