In this article I am going to treat a group of words of the Finno-Ugric languages, which in all my opinion belong to the same mythological context: the so-called twin cult, which is more or less independently known in most of the cultures. The main question in this analysis is, whether it is important to know the cultural background in order to establish an etymology - an old question, which the answer is no in classic Finno-Ugrian studies: although the knowledge of the backgrounds is theoretically considered to be important, in general an etymology, which does not treat any cultural considerations, is judged to be better than an etymology that does. The first part treats the Finno-Ugric relatives of Estonian jama, jäma 'joint; space between two parts belonging together'. The second part is about the Ugric forms that are related to Hungarian jó 'good'. In this context I am going to refer to the etymological and mythological backgrounds of the Ugrian form *manc<. Finally an Ob-Ugric word family will be discussed. All the words under consideration are closely related to (Indo-)Iranian mythological conceptions.