The English language was first introduced to the Americas by British colonisation, beginning in the late 16th and early 17th centuries. The language also spread to numerous other parts of the world as a result of British trade and colonisation and the spread of the former British Empire, which, by 1921, included about 470–570 million people, about a quarter of the world’s population. Written forms of British and American English as found in newspapers and textbooks vary little in their essential features, with only occasional noticeable differences.
Over the past 400 years, the forms of the language used in the Americas—especially in the United States—and that used in the United Kingdom have diverged in a few minor ways, leading to the versions now often referred to as American English and British English. Differences between the two include pronunciation, grammar, vocabulary (lexis), spelling, punctuation, idioms, and formatting of dates and numbers. However, the differences in written and most spoken grammar structure tend to be much less than in other aspects of the language in terms of mutual intelligibility.
This divergence between American English and British English has provided opportunities for humorous comment: e.g. in fiction George Bernard Shaw says that the United States and United Kingdom are “two countries divided by a common language”; and Oscar Wilde says that “We have really everything in common with America nowadays, except, of course, the language” (The Canterville Ghost, 1888).
Henry Sweet incorrectly predicted in 1877 that within a century American English, Australian English and British English would be mutually unintelligible (A Handbook of Phonetics)
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The Lectures 