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We establish several facts about medieval monetary debasements: they were followed by unusually large minting volumes and by increased seigniorage; old and new coins circulated concurrently; and, at least some of the time, coins were valued by weight. These facts constitute a puzzle because debasements provide no additional inducements to bring coins to the mint. On theoretical and empirical grounds, we reject explanations based on by-tale circulation, nominal contracts, and sluggish price adjustment. We conclude that debasements pose a challenge to monetary economics.