Bicategories of relations or spans have attracted many category theorists since the 1970s, and the study has been extended to the world of double categories in recent years. In this talk, I will provide an overview of the development of this field, particularly in relation to a condition called the Frobenius axiom, as presented in Walters and Wood’s paper “Frobenius objects in Cartesian bicategories.” This axiom can be understood as a criterion for a double category to qualify as a “double category of relations,” and I will explain some results that support this idea.