In the 16th century, foreign ships passing through the Sound had to pay ad valorem taxes, known as the Sound Dues. To give skippers an incentive to declare the true value of their cargo, the Danish Crown reserved the right to purchase it at the declared value. We show that this rule does not induce truth-telling, but does allow the authorities to effectively implement a given tax rate. (C) 2011 Elsevier B.V. All rights reserved.