Ring languages exist too, also called language continua. I'm thinking of German, for example, not the book German people learn in school, but the dialects they speak at home, even though this phenomenon is now dying out. Language continua are easier to understand in historical contexts from times before national languages began to be enforced via state encouragement and power. But anyway, the German that people speak at home in southern Germany, like in Munich and Augsburg, is quite similar but not identical to the German spoken at home in nearby areas, like in Austria and a little further north in Germany. But the further north you go, the more differences accumulate, until eventually, you arrive in say, Cologne or Berlin, where the local dialects are not very mutually comprehensible with what's spoken down south. Somebody from Cologne pretty much has to speak to somebody from Augsburg in standard book German, because when they try to speak to each other in their cradle language, they can't understand each other.

Yet at no time while traveling would you find residents of neighboring towns who can't understand one another in their cradle language.

Linguists say this causes them no ends of problems categorizing languages objectively, given that most people (who don't study linguistics) WANT to use mutual comprehensibility as the gold standard of defining a language. It feels quite natural to do so, and that definition is useful most of the time.

But it's not useful all the time, especially to linguists who are very interested in grey areas. Is the language spoken in Munich homes the same as the language spoken in Berlin homes? Are both languages German? Yes, they'll say, but probably not the way you think we mean. It's complicated.