Using new sources, this article examines how in the years around 1550 Charles V and the imperial estates came close to creating a common currency for the Holy Roman Empire. The article analyses whose interests prevailed in the negotiations and how, despite the resistance of some important actors, the Imperial diet of 1551 was able to unanimously agree on the currency bill. It also analyses why the common currency still failed: This was the case because of the desire of many princes to ease the repayment of their debts by establishing a bimetallic currency, and even more importantly because of Charles V's attempt to weaken the Elector of Saxony by undervaluing the taler. In this, Charles exploited the diet's implicit ex‐ante agreement with him to set the rates at which old money was allowed to continue in circulation. His manipulations provoked resistance, raised the costs of implementing the common currency, and caused its failure.