The homogeneity of linguistic community is a useful pragmatic assumption, at least at a certain point of linguistic research. If languages are, first and foremost, instruments of communication, it is fairly natural that we should assume, at least as an ideal, that all people who use one of them share the bundles of articulatory habits and vocal reactions to various stimuli whose sum total we call language: communication would be best secured if all people concerned spoke exactly in the same way.
- Martinet (1962), a pag.104

We can conclude from all this that the notion of linguistic community is not only useful, but unavoidable in linguistics as soon as language is conceived as an instrument of communication constantly adapting itself to the needs of the group who make use of it; ‘comunication’ implies ‘community’ […] 1. No community is linguistically homogeneous: no two person use a language in exactly the same way; the same situation will elicit different linguistic reactions from different onlookers; no two person will use or understand the very same vocabulary; even the highly structured aspects of language, such as phonology and morphology, may differ in important matters from on speaker to another without impairing mutual understanding and even without being noticed by interlocutors. 2. Many people belong to two, or more than two, communities.
- Martinet (1962), a pag.107-108