The debate on the conventions of the constitution has led to a variety of tools for analysing institutional practices in national governments. Few attempts have been made to identify relevant political rules in the EU's ‘form of government’. This paper argues the applicability of the conventional theory in understanding the EU's most recent constitutional developments by comparing the common law and continental understandings of conventions. It highlights the different features attached to conventions in various national experiences and offers a fresh view of them to take the most out of this notion, long forlorn in EU studies. Using three practical experiences (trilogues, parliamentary hearings for Commissioners-designate and the Spitzenkandidaten practice), the paper shows how unwritten rules can shape the EU's political or material constitution up to the point of bringing concrete informal changes without any formal Treaty amendment.