Updated 03-Aug-2025
Mutual Intelligibility is the ability for two speakers (or writers) of different languages, to understand each other. Certainly there are degrees when it comes to mutual intelligibility, and not simply intelligibility or not. And there are many ways of measuring mutual intelligibility, with the sentence-level utterance a key feature.
On the one hand there is the language Hindustani comprised of Hindi and Urdu. Both of which for many intents and purposes are a single language, though of course now divided politically, religiously, and by new vocabulary. The same is true of Romanian and Moldovan, though to a lesser degree. Then we have the wide variety of Languages in China, indeed mutual intelligibility is fairly weak overall.
Simply not useful when trying to understand language, but extremely clear-sighted when trying to understand how Mandarin is supposedly a language spoken by some 800 million to over 1 billion people.
In reality, even with the closely associated so-called Mandarin group of languages, and including those languages with a somewhat low (though still existent) intelligibility, we are talking about a few hundred million at most of a single language's speakers.
Symmetric and Asymmetric Intelligibility
Interestingly, intelligibility is not always a two-way street. One can understand the utterances and writings of another, whereas the second interlocutor may not understand as much, or may understand more than the first. Also interesting is the fact that with the communicative approach to foreign language learning, mutual intelligibility is actually the goal, rather than language mastery.