It is therefore sufficient to define the areas covered by these dialects in quite a rough way.

We may regard the Northumbrian or Northern as the dialect or group of dialects spoken to the north of the river Humber, as the name implies; the Wessex or Southern, as the dialect or group of dialects spoken to the south of the river Thames; the Kentish as being peculiar to Kent; and the Mercian as in use in the Midland districts, chiefly to the south of the Humber and to the north of the Thames.

The modern limits are somewhat different, but the above division of the three chief dialects (excluding Kentish) into Northern, Midland, and Southern is sufficient for taking a broad general view of the language in the days before the Norman Conquest.
The investigation of the differences of dialect in our early documents only dates from 1885, owing to the previous impossibility of obtaining access to these oldest texts.

Before that date, it so happened that nearly all the manuscripts that had been printed or examined were in one and the same dialect, viz.

the Southern (or Wessex).