This chapter explores the growing use of English as a written ‘legal vernacular’ over the course of the fifteenth century. It argues that one can only understand the emergence of vernacular writing in legal discourse by looking to the local contexts of legal production. The emergence of English as a legal vernacular did not take hold uniformly across late-medieval society, and so we need to think more carefully about the specific kinds of discursive value that it held; the chapter argues that, as a legal language, English worked as a signifier of authenticity, a mode of signalling fidelity to real speech, and as a way of gesturing towards wider audiences or publics. This leads to the third argument that the growing significance granted to English as a legal language affected common people in late-medieval England in ambivalent ways. While in some ways the processes of vernacularization in the fifteenth century seem to follow a trajectory towards a more inclusive public discourse, as the ‘common tongue’ spoken by the majority of the populace became a language appropriate for expressing ideas about legitimacy, it was ultimately constrained by the relatively limited modes in which English was allowed to be legal.