Old English should not be regarded as a single monolithic entity, just as Modern English is also not monolithic.
It emerged over time out of the dialects of the colonising tribes, and it is only towards the later Anglo-Saxon period that these can be considered to have constituted a single national language.
Even then, Old English continued to exhibit much local and regional variation, remnants of which remain in Modern English dialects.

The four main dialectal forms of Old English were Mercian, Northumbrian, Kentish, and West Saxon.
Mercian and Northumbrian are together referred to as Anglian.
In terms of geography the Northumbrian region lay north of the Humber River; the Mercian lay north of the Thames and South of the Humber River; West Saxon lay south and southwest of the Thames; and the smallest, Kentish region lay southeast of the Thames, a small corner of England.
The Kentish region, settled by the Jutes from Jutland, has the scantest literary remains.

Each of these four dialects was associated with an independent kingdom on the island.
Of these, Northumbria south of the Tyne, and most of Mercia, were overrun by the Vikings during the 9th century.
The portion of Mercia that was successfully defended, and all of Kent, were then integrated into Wessex under Alfred the Great.

From that time on, the West Saxon dialect (then in the form now known as Early West Saxon) became standardised as the language of government, and as the basis for the many works of literature and religious materials produced or translated from Latin in that period.

The later literary standard known as Late West Saxon (see History, above), although centred in the same region of the country, appears not to have been directly descended from Alfred's Early West Saxon.
For example, the former diphthong /iy/ tended to become monophthongised to /i/ in EWS, but to /y/ in LWS.

Due to the centralisation of power and the Viking invasions, there is relatively little written record of the non-Wessex dialects after Alfred's unification.
Some Mercian texts continued to be written, however, and the influence of Mercian is apparent in some of the translations produced under Alfred's programme, many of which were produced by Mercian scholars.
Other dialects certainly continued to be spoken, as is evidenced by the continued variation between their successors in Middle and Modern English.
In fact, what would become the standard forms of Middle English and of Modern English are descended from Mercian rather than West Saxon, while Scots developed from the Northumbrian dialect.
It was once claimed that, owing to its position at the heart of the Kingdom of Wessex, the relics of Anglo-Saxon accent, idiom and vocabulary were best preserved in the dialect of Somerset.

For details of the sound differences between the dialects, see Phonological history of Old English § Dialects.
Old English should not be regarded as a single monolithic entity, just as Modern English is also not monolithic. It emerged over time out of the dialects of the colonising tribes, and it is only towards the later Anglo-Saxon period that these can be considered to have constituted a single national language. Even then, Old English continued to exhibit much local and regional variation, remnants of which remain in Modern English dialects.
The four main dialectal forms of Old English were Mercian, Northumbrian, Kentish, and West Saxon. Mercian and Northumbrian are together referred to as Anglian. In terms of geography the Northumbrian region lay north of the Humber River; the Mercian lay north of the Thames and South of the Humber River; West Saxon lay south and southwest of the Thames; and the smallest, Kentish region lay southeast of the Thames, a small corner of England. The Kentish region, settled by the Jutes from Jutland, has the scantest literary remains.
Each of these four dialects was associated with an independent kingdom on the island. Of these, Northumbria south of the Tyne, and most of Mercia, were overrun by the Vikings during the 9th century. The portion of Mercia that was successfully defended, and all of Kent, were then integrated into Wessex under Alfred the Great.
From that time on, the West Saxon dialect (then in the form now known as Early West Saxon) became standardised as the language of government, and as the basis for the many works of literature and religious materials produced or translated from Latin in that period.
The later literary standard known as Late West Saxon (see History, above), although centred in the same region of the country, appears not to have been directly descended from Alfred's Early West Saxon. For example, the former diphthong /iy/ tended to become monophthongised to /i/ in EWS, but to /y/ in LWS.
Due to the centralisation of power and the Viking invasions, there is relatively little written record of the non-Wessex dialects after Alfred's unification. Some Mercian texts continued to be written, however, and the influence of Mercian is apparent in some of the translations produced under Alfred's programme, many of which were produced by Mercian scholars. Other dialects certainly continued to be spoken, as is evidenced by the continued variation between their successors in Middle and Modern English. In fact, what would become the standard forms of Middle English and of Modern English are descended from Mercian rather than West Saxon, while Scots developed from the Northumbrian dialect. It was once claimed that, owing to its position at the heart of the Kingdom of Wessex, the relics of Anglo-Saxon accent, idiom and vocabulary were best preserved in the dialect of Somerset.
For details of the sound differences between the dialects, see Phonological history of Old English § Dialects.
Old English