This paper presents novel evidence for Polish nominals being phases. The core of the argument comes from the interaction between the size of a nominal domain and the corresponding person-feature valuation. While an nP-size nominal structure only exhibits feature valuation within the narrow-syntax module, the DP-size nominal structure exhibits interface feature interactions. Since interface interactions can only arise during spell-out, the corresponding nominal domain, i.e., DP, must be a phase and D must be a phase head that triggers spell-out. The empirical focus of this paper is on a syntactic variation in the domain of politeness markers in Polish (pan.m/ pani.f).