How designers communicate within design teams, and with users, suppliers, and customers, differs in formality both between industries and between different situations within one project. This paper identifies three layers of structure in design communication, each of which can be more or less formal: the design process, the interaction between participants, and the representations of design information that are constructed and used. These layers can be formal across a spectrum from explicit rules to habitual conventions. The paper draws on a range of contrasting case studies in mechanical engineering and knitwear design, as well as a larger corpus of cases comparing design domains more generally, to analyze how formality affects design interaction in different situations and process contexts. Mismatches in the understanding of formality can lead to misunderstandings, in particular across expertise boundaries and between designers and their clients or customers. Formality can be modulated in the mannerism of communication, the rhetoric employed, and how representations are constructed, to make communication more effective. The effort and skill put into modulating formality is greater in domains where designers work with end users, like architecture, than it is in companies where designers interact mainly with other professionals.