Insofar as it is possible to draw a line between languages and dialects, linguists generally do so based on mutual intelligibility: if two idioms are mutually intelligible, they’re dialects; if they’re not, they’re separate languages. But a situation can also arise where a speaker of one idiom understands the second, but the second doesn’t understand the first; this is called asymmetric intelligibility. For example:
- Portuguese speakers can usually understand Spanish, but Spanish speakers cannot usually understand Portuguese (at least spoken – written is generally very intelligible)
- Norwegians have an easier time understanding Swedish and Danish than the inverse, and Swedes have a very difficult time trying to understand Danish even though Danes don’t struggle as much with Swedish
- Jamaican Patois speakers understand English, but most native English speakers struggle greatly to understand Jamaican Patois
There are three main reasons why intelligibility might be asymmetrical. The first is, of course, the languages themselves – e.g. where two (or more) languages are very closely related, but one has been very innovative with the phonology (like Portuguese or Danish) while the other has been more conservative and corresponds more closely to the orthography (like Spanish or Swedish). But there are two other important reasons: exposure (like where, for example, Jamaicans are exposed to way more “standard English” than speakers of “standard English” are to Jamaican patois) and motivation (that is, if you want the conversation to be successful you work harder to understand, and if you don’t you don’t).