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Each of the maps in a linguistic atlas will explore the distribution of a different variant (a sound, a word, a phrase) using a sequence of points on a map

An isogloss is like a variant boundary. It’s a line drawn on the map to show where a linguistic variant is found.

https://en.wikipedia.org/wiki/Isogloss

When several of these lines intersect, you can consider it a dialect boundary. I.e. all these features together are what give you a specific dialect like , for example.