The way I've heard it explained (and I only studied Mandarin for a year) is that a common language and writing can be simultaneously compatible and incompatible

The way I've heard it explained (and I only studied Mandarin for a year) is that a common language and writing can be simultaneously compatible and incompatible. Consider these stereotypes - an urban youth from inner city America and a gruff old Yorkshireman. Both speak English and use the same alphabet and mostly the same words. Yet the syntax and slang differences each have could render the dialects meaningless to each other. But, write down what they are saying and suddenly the meaning becomes a lot clearer. Yes, there are differences, but you can probably work your way around them.