The focus of this paper is on speakers' rationalisations of their everyday linguistic choices as members of a multilingual academic department in the US. Given the monolingual macro-context, the myriad of native languages spoken by participants, and the professional stake in language competence, the question of how speakers arrive at language choices is examined through in-depth interviews with participants (n = 12). Results indicate that participant dyads characteristically have a dominant language, but occasionally switch to the non-dominant variety. While most of the factors and motivations speakers identify can be situated in existing frameworks for code choice, my central argument is that community-wide explanations are inadequate for this group of speakers. Rather, I argue that micro-norms characterising interlocutor dyads emerge through the negotiation of perceived norms, cultural beliefs and language preferences. An important feature of dyadic norms is their mutability, within situations as well as across time. (Contains 2 notes and 2 tables.)