Map 5 shows that the Western New England TLN region corresponds very neatly to the region possessing both of the features that I argue promote TLN on Maps 3 and 4: rhoticity and monophthongal /ehr/.
The dotted green line indicates the area that is within both the green isogloss of Map 3 and the yellow isogloss of Map 4. Except for the southern N.H./Maine TLN area, which I argue has a more social than necessarily phonological motivation, the dotted green line approximates the red TLN isogloss fairly well. We see a few places, especially in Connecticut, where TLN has spread eastward of the phonological factors that favor it; we can regard this as an illustration of the principle that mergers tend to expand at the expense of distinctions.
Map 5: TLN versus the combination of the isoglosses from Maps 3 and 4.