In dialectology, the study of dialects, the notion of mutual intelligibility has been used as the most reliable criterion to determine whether two linguistic communities speak different languages or different dialects of the same language.

If two people are speaking different languages, they probably cannot understand each other. In this case, the different languages being spoken are not mutually intelligible. Thus, an Indonesian speaking Bahasa Indonesia (henceforth BI) will not probably be understood by a Chinese speaking Mandarin, and vice versa.

However, two people speaking distinct language varieties of the same language can normally understand each other very well. According to the mutual intelligibility criterion, people speaking Toronto English will have no difficulties in understanding those speaking Australian English and American English.