Kentish was originally spoken over the whole southeastern part of England, including London and Essex, but during the Middle English period its area was steadily diminished by the encroachment of the East Midland dialect, especially after London became an East Midland-speaking city (see below); in late Middle English the Kentish dialect was confined to Kent and Sussex. In the Early Modern period, after the London dialect had begun to replace the dialects of neighboring areas, Kentish died out, leaving no descendants. Kentish is interesting to linguists because on the one hand its sound system shows distinctive innovations (already in the Old English period), but on the other its syntax and verb inflection are extremely conservative; as late as 1340, Kentish syntax is still virtually identical with Old English syntax.
The Southern dialect of Middle English was spoken in the area west of Sussex and south and southwest of the Thames. It was the direct descendant of the West Saxon dialect of Old English, which was the colloquial basis for the Anglo-Saxon court dialect of Old English. Southern Middle English is a conservative dialect (though not as conservative as Kentish), which shows little influence from other languages — most importantly, no Scandinavian influence (see below). Descendants of Southern Middle English still survive in the working-class country dialects of the extreme southwest of England.
By contrast with these southernmost dialects, Northern Middle English evolved rapidly: the inflectional systems of its nouns and verbs were already sharply reduced by 1300, and its syntax is also innovative (and thus more like that of Modern English). These developments were probably the result of Scandinavian influence. In the aftermath of the great Scandinavian invasions of the 860's and 870's, large numbers of Scandinavian families settled in northern and northeastern England. When the descendants of King Alfred the Great of Wessex reconquered those areas (in the first half of the 10th century), the Scandinavian settlers, who spoke Old Norse, were obliged to learn Old English. But in some areas their settlements had so completely displaced the preexisting English settlements that they cannot have had sufficient contact with native speakers of Old English to learn the language well. They learned it badly, carrying over into their English various features of Norse (such as the pronoun they and the noun law ), and also producing a simplified syntax that was neither good English nor good Norse. Those developments can be clearly seen in a few late Old English documents from the region, such as the glosses on the Lindisfarne Gospels (ca. 950) and the Aldbrough sundial (late 11th century). None of this would have mattered for the development of English as a whole if the speakers of this "Norsified English" had been powerless peasants; but they were not. Most were freeholding farmers, and in many northern districts they constituted the local power structure. Thus their bad English became the local prestige norm, survived, and eventually began to spread (much later — see below).
The East-Midland and West-Midland dialects of Middle English are intermediate between the Northern and Southern/Kentish extremes. In the West Midlands there is a gradation of dialect peculiarities from Northern to Southern as one moves from Lancashire to Cheshire and then down the Severn valley. This dialect has left modern descendants in the working- class country dialects of the area. The East-Midland dialect is much more interesting. The northern parts of its dialect area were also an area of heavy Scandinavian settlement, so that northern East-Midland Middle English shows the same kinds of rapid development as its Northern neighbor. But the subdialect boundaries within East-Midland were far from static: the more northerly variety spread steadily southward, extending the influence of Scandinavianized English long after the Scandinavian population had been totally assimilated. In the 13th century this part of England, especially Norfolk and Suffolk, began to outstrip the rest of the country in prosperity and population because of the excellence of its agriculture, and — crucially — increasing numbers of well-to-do speakers of East-Midland began to move to London, bringing their dialect with them. By the second half of the 14th century the dialect of London and the area immediately to the northeast, which had once been Kentish, was thoroughly East-Midland, and a rather Scandinavianized East Midland at that. Since the London dialect steadily gained in prestige from that time on and began to develop into a literary standard, the northern, Scandinavianized variety of East-Midland became the basis of standard Modern English. For that reason, East-Midland is by far the most important dialect of Middle English for the subsequent development of the language.