The study of constitutional conventions is anchored in three assumptions that have so far remained largely unchallenged: that there is a shared “Commonwealth approach” to constitutional conventions; that Commonwealth courts will recognize and employ conventions but never enforce them; and that conventions are always distinguishable from rules of law. After setting out a new taxonomy of modes of judicial engagement with constitutional conventions, we overturn each of these assumptions. We draw on recent case law from Canada, India, and the United Kingdom to show that there is no shared “Commonwealth approach” to the treatment of constitutional conventions. We show that some Commonwealth courts do, in fact, enforce conventions. Finally, we show that at least some constitutional conventions have crystallized into law. These insights disrupt much of what is foundational in the study of constitutional conventions.