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. Hence, the term isogloss, from iso - ` same ' + gloss ` word, ' to describe the line on a map where the terms are of equal distribution.
Isoglosses, in effect, set off areas where particular usage was recorded. More accurately, an isogloss is drawn to connected sites where speakers employ both usages or where speakers live in very close proximity.