Modern linguistic theories try to give an exhaustive explanation of how language works. In this perspective, each linguistic domain, such as phonetics, phonology, syntax, pragmatics, etc., is described by means of a set of rules or properties (in other words, a grammar). However, it is a long time linguists have observed that it is not possible to give a precise description of a real life utterance within a unique domain. We illustrate this problem with the case of phonetic variability and show how different domains interact. We propose then a two-level architecture in which domain interaction is implemented by means of constraints.