Legal designers use different mechanisms to entrench constitutions. This article studies one mechanism that has received little attention: constitutional “locks,” or forced waiting periods for amendments. We begin by presenting a global survey, which reveals that locks appear in sixty-seven national constitutions. They vary in length from nine days to six years, and they vary in reach, with some countries “locking” their entire constitution and others locking only select parts. After presenting the survey, we consider rationales for locks. Scholars tend to lump locks with other tools of entrenchment, such as bicameralism and supermajority rule, but we argue that locks have distinct and interesting features. Specifically, we theorize that locks can cool passions better than other entrenchment mechanisms, promote principled deliberation by placing lawmakers behind a veil of ignorance, and protect minority groups by creating space for political bargaining. Legislators cannot work around locks, and because locks are simple and transparent, lawmakers cannot “break” them without drawing attention. For these reasons, we theorize that locks facilitate constitutional credibility and self-enforcement, perhaps better than other entrenchment mechanisms.