One report explains the geographic difference in detail:This distinction was first recognised formally at the turn of the 19th / 20th centuries, when linguists drew an isophone diagonally across the county from the northwest to the southeast, separating these two broadly distinguishable ways of speaking. It can be extended westwards through Lancashire to the estuary of the River Lune, and is sometimes called the Humber-Lune Line. Strictly speaking, the dialects spoken south and west of this isophone are Midland dialects, whereas the dialects spoken north and east of it are truly Northern. It is possible that the Midland form moved up into the region with people gravitating towards the manufacturing districts of the West Riding during the Industrial Revolution.
One report explains the geographic difference in detail:This distinction was first recognised formally at the turn of the 19th / 20th centuries, when linguists drew an isophone diagonally across the county from the northwest to the southeast, separating these two broadly distinguishable ways of speaking. It can be extended westwards through Lancashi:re to the estuary of the River Lune, and is sometimes called the Humber-Lune Line. Strictly speaking, the dialects spoken south and west of this isophone are Midland dialects, whereas the dialects spoken north and east of it are truly Northern. It is possible that the Midland form moved up into the region with people gravitating towards th?e manufacturing districts of the West Riding during the Industrial Revolution.