The late medieval and early modern periods saw a fairly steady process of standardization in English south of the Scottish border. The written and spoken language of London continued to evolve and gradually began to have a greater influence in the country at large. For most of the Middle English period a dialect was simply what was spoken in a particular area, which would normally be more or less represented in writing - although where and from whom the writer had learnt how to write were also important. It was only when the broadly London standard began to dominate, especially through the new technology of printing, that the other regional varieties of the language began to be seen as different in kind. As the London standard became used more widely, especially in more formal contexts and particularly amongst the more elevated members of society, the other regional varieties came to be stigmatized, as lacking social prestige and indicating a lack of education.
In the same period a series of changes also occurred in English pronunciation (though not uniformly in all dialects), which go under the collective name of the Great Vowel Shift. These were purely linguistic ‘sound changes’ which occur in every language in every period of history. The changes in pronunciation weren’t the result of specific social or historical factors, but social and historical factors would have helped to spread the results of the changes. As a result the so-called ‘pure’ vowel sounds which still characterize many continental languages were lost to English. The phonetic pairings of most long and short vowel sounds were also lost, which gave rise to many of the oddities of English pronunciation, and which now obscure the relationships between many English words and their foreign counterparts.