2. A dialect continuum is a situation where a language changes slowly into another one over geographical distance. For example, there is no clear cutoff line (isogloss) between German and Dutch. Germans near the border speak the same as Dutch people near the border, and as one travels closer to Berlin, it becomes more German; and as one travels towards Amsterdam, the language becomes more Dutch.
3. A dialect boundary occurs where there is a clear geographical boundary - the opposite of 2. Basically, it's the same as an isogloss.
For example, there is an isogloss between France and England - the English Channel. In China, there are many languages, and language or dialect continua are more common.
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