The Germanic tribes who settled in Britain in the 5th and 6th c. spoke closely related tribal dialects belonging to the West Germanic subgroup. Their common origin and their separation from other related tongues as well as their joint evolution in Britain transformed them eventually into a single tongue, English. The OU dialects acquired certain common features which distinguished them from continental Germanic tongues. Also they displayed growing regional divergence. Tribal dialects were transformed into local or regional dialects. The following four principal OE dialects are commonly distinguished: Kentish, a dialect spoken in the area known now as Kent and Surrey and in the Isle of Wight. It had developed from the tongue of the Jutes and Frisians. West Saxon, the main dialect of the Saxon group, spoken in the rest of England south of the Thames and the Bristol Channel, except Wales and Cornwall, where Celtic tongues were preserved. Other Saxon dialects in England have not survived in written form and are not known to modern scholars. Mercian, a dialect derived from the speech of southern Angles and spoken chiefly in the kingdom of Mercia, that is, in certain region, from the Thames to the Humber. Nothumbrian, another Anglian dialect, spoken from the Humber north to the river Forth. The boundaries between the dialects were uncertain and probably movable. The dialects passed into one another imperceptibly and dialectal forms were freely borrowed from one dialect into another. Throughout this period the dialects enjoyed relative equality; none of them was the dominant form of speech, each being the main type used over a limited area. At the time of written OE the dialects had changed from tribal to regional; they possessed both an oral and a written form and were no longer equal; in the domain of writing the West Saxon dialect prevailed over its neighbours.
In the 9th c. the political and cultural centre moved to Wessex. Culture and education made great progress there; it is no wonder that the West Saxon dialect has been preserved in a greater number of texts than all the other OE dialects put together. Towards the 11th c. the written form of the West Saxon dialect developed into a bookish type of language, which, probably, served as the language of writing for all English-speaking people.