The article deals with the conditions that did not have the legal effect of a ‘proper condition’. The authors distinguish these conditions from a ‘proper condition’ pointing out that the distinguishing feature is an ‘uncertainty’ carried by the condition. Firstly, the authors focus their attention on the condicio supervacua – the term is explicitly used only by Pomponius in the case of legacies. It did not have the effect of a ‘proper condition’, because the uncertainty expressed by the condition already resulted from the legal norm itself. It was an explicitly expressed condicio iuris pursuant to the modern definitions of the term. The authors analyze and compare various cases that seemingly deal with condiciones iuris too. The analysis makes it clear that they were regarded as supervacuae, unless the testator changed the legal situation in some way through their expression. He had to insert some new uncertainty that did not result from the legal norm itself. The other kind of condition that the article deals with is condicio institutionis/substitutionis expressly re-applied to a legatum. This condition is different from condicio supervacua, because in this case the new uncertainty was added by the testator himself. Despite some doubts, the legal opinion which prevailed in Roman law was that such a condition did not have the effect of a ‘proper condition’ in relation to a legacy. The reason is that even if the testator re-applied the condition of institution also to a legacy, he extended only the uncertainty of aditio hereditatis and did not insert any new uncertainty into the legacy itself.