One of my Linguistics professors used to say that the difference between a dialect and a language was that the language was backed up by a government, borders, and an army. Thus you get, for example, several Scandinavian languages that are mutually comprehensible - say you have countries lined up like A-B-C-D-E. People in one country can easily understand people from a country that's right next to them, and kind of understand people from the next country along, which means that linguistically speaking, they are not separate languages; they are dialects of one language, or on a gradient from language A to language E. However, the people speaking those languages have national pride (and borders, and armies) and insist that no, they have their OWN language thank you, so culturally they are considered separate.
Chinese is the opposite situation. :) Speakers of Mandarin and Cantonese can't understand each other; they share a lot of vocabulary, but then so do an ancient form of Chinese and Japanese and nobody tries to call those dialects of the same language. ;) They have different pronunciation, different grammatical structure, verbs go in a different place in the sentence and so on. To a linguist, therefore, they are completely separate languages, and each has numerous regional dialects just to complicate matters. Written down, however, they're essentially identical, and they share the government/border/army, so are often - culturally speaking - regarded as two dialects of the same language, which makes linguists wince and mutter things like "Well, not really..."
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One of my Linguistics professors used to say that the difference between a dialect and a language was that the language was backed up by a government, borders, and an army. Thus you get, for example, several Scandinavian languages that are mutually comprehensible - say you have countries lined up like A-B-C-D-E. People in one country can easily understand people from a country that's right next to them, and kind of understand people from the next country along, which means that linguistically speaking, they are not separate languages; they are dialects of one language, or on a gradient from language A to language E. However, the people speaking those languages have national pride (and borders, and armies) and insist that no, they have their OWN language thank you, so culturally they are considered separate.
Chinese is the opposite situation. :) Speakers of Mandarin and Cantonese can't understand each other; they share a lot of vocabulary, but then so do an ancient form of Chinese and Japanese and nobody tries to call those dialects of the same language. ;) They have different pronunciation, different grammatical structure, verbs go in a different place in the sentence and so on. To a linguist, therefore, they are completely separate languages, and each has numerous regional dialects just to complicate matters. Written down, however, they're essentially identical, and they share the government/border/army, so are often - culturally speaking - regarded as two dialects of the same language, which makes linguists wince and mutter things like "Well, not really..."