Where more than 200 Chinese dialects are spoken, how is China united by ‘common speech’?

Over 90 per cent of people in China speak regional forms of Chinese, commonly referred to as dialects, which can be as mutually unintelligible as English and German. Speakers living just hours away from each other may not be able to understand each other at all. But most of the population can still communicate using the country’s “common speech” or Putonghua. So, how did China unify its spoken language?