Although it is true that isoglosses, in effect, set off the various areas where a particular usage was recorded, more accurately an isogloss is drawn to connect sites either where speakers employ both usages or where speakers using one or the other live in very close proximity.
Isoglosses set off the areas where a particular usage was recorded. More accurately, an isogloss is drawn to connect sites where speakers employ both usages or where speakers live in very close proximity.