From The Art and Popular Culture Encyclopedia
Middle English is the name given by historical linguistics to the diverse forms of the English language spoken between the Norman invasion of 1066 and about 1470, when the Chancery Standard, a form of London-based English, began to become widespread, a process aided by the introduction of the printing press into England by William Caxton in the 1470s, and slightly later by Richard Pynson. By this time the Northumbrian dialect (prevalent in Northern England) spoken in southeast Scotland was developing into the Scots language. The language of England as spoken after this time, up to 1650, is known as Early Modern English.
Unlike Old English, which tended largely to adopt Late West Saxon scribal conventions in the immediate pre-Conquest period, Middle English as a written language displays a wide variety of scribal (and presumably dialectal) forms. However, the diversity of forms in written Middle English signifies neither greater variety of spoken forms of English than could be found in pre-Conquest England, nor a faithful representation of contemporary spoken English (though perhaps greater fidelity to this than may be found in Old English texts). Rather, this diversity suggests the gradual end of the role of Wessex as a focal point and trend-setter for scribal activity, and the emergence of more distinct local scribal styles and written dialects, and a general pattern of transition of activity over the centuries that follow, as Northumbria, East Anglia and London emerge successively as major centres of literary production, with their own generic interests.