2. Many people belong to two, or more than two communities. This is, of course, the case in such well-known bilingual stretches as Brussels, Alsace, or South Africa. But this applies to many situation where both a vernacular and a standard language are alternately used by the same people with different interlocutors. From a linguistic standpoint we cannot make bilingualism depend on the amount of prestige enjoyed by the two forms of speech in contact. The alternate use of different phonological systems is probably the least ambiguous test of a bilingual situation.
- Martinet (1962), a pag.108

The only valid criterion in such matters is less perhaps the ease of understanding, which we would not know how to misure, than the existence of bilingualism, in the widest sense of the word, namely, the use by the same persons of two different phonological and morphological patterns depending on one’s interlocutors.
- Martinet (1962), a pag.111-112