ABSTRACT The paper studies how communicative roles and registers are expressed in an interde pendent way, influencing each other in current forums. In the first part, by combining previous insights in register variation analysis, social identity theory and cognitive linguis tics, three new concepts for linguistic analysis are proposed: 'Register-Modulating Role' (RMR), 'Register compass' (R-compass) and 'pivotal register'. The way they function in digital discourse is illustrated in the second part by analysing an open discussion forum sample corpus. The results show that each forum's profile displays a different array of RMR, together with their associated registers, which tend to spin around a prominent pivotal register. In our corpus three salient pivotal registers are identified, together with their associated registers and distinctive language features. The study substantiates the close connection between relational identities or roles and registers in digital discourse, and facilitates linguistic devices for a better use and understanding of register variation and forum discourse.