What does an isogloss show on a dialect map?

On a dialect map, an isogloss represents the boundary line between regions that differ in a specific linguistic feature.12

Every individual linguistic phenomenon—such as a specific pronunciation, grammatical structure, or vocabulary choice—possesses its own unique isogloss. Because these features rarely change in perfect unison, individual isoglosses often cross and interweave to form complex patterns.2

When several isoglosses group together, they form a "bundle of isoglosses." These bundles often signify more significant divisions, such as those caused by geographic barriers, long-standing political borders, or historical migrations that brought distinct populations into contact. By identifying these bundles or particularly prominent individual isoglosses, linguists can define and divide territories into distinct regional dialects.2