This article addresses how mediated discourse theory and related analytical tools can be used to explore how students write together. Considered within a sociocultural framework that conceptualises writing as involving distributed, mediated and dialogic processes of invention, this article presents an investigation of how three high school students wrote together for a collaborative project. This article presents a writing event selected from a larger study that is used to explore the ways authorship is distributed among the students, the resources that mediate their writing, the shifting social contexts they establish when writing and the relational and reflexive social positioning they enact. Mediated discourse theory and its related analytical tools are introduced for heuristic and methodological purposes for analysing how the coordination of these complexities shapes students' writing. The purpose for doing so is to gain a better understanding of how and why students write in the ways they do.