While defendants can always appeal convictions in trial, public prosecutors often face prohibition of appealing acquittals. This asymmetry twists the criminal procedure towards the interests of defendants. The paper first inquires the impact of asymmetrical appeal powers on the number of convictions, acquittals and errors of type I and type II. It then surveys the traditional justifications of asymmetric appeal powers. Finally it frames pro-defendant procedural safeguards, such as asymmetric appeal powers, into the Blackstonian precept that it is “better that ten guilty escape than that one innocent should suffer” (Blackstone, 1766) (J.E.L: K4) “Nor shall any person be subject for the same offence to be twice put in jeopardy of life or limb”. These are the words of the US constitution (fifth amendment), which most scholars refer to when discussing the impossibility of appealing acquittals by public prosecutors in criminal cases. This impossibility introduces a neat asymmetry in the procedure between the prosecutor and the defendant as the latter has much larger chances of appealing a conviction whereas the former can hardly obtain retrial after an acquittal. This doctrine is known in the US as double jeopardy. Indeed double jeopardy encompasses a larger set of rules meant at protecting the defendant against retrial, and in general, double jeopardy must be seen in the context of a range of pro-defendant devices that characterize the criminal procedure across jurisdictions and that have been consistently applied over time. These safe