In Section 2 [001C], we analyse the problem of exact grids, and in Section 3 [001D] we indicate a possible solution to this problem through “relational calculi”; we observe that all the different ways to approach the calculus of relations (Bénabou, Guitart, Hilton, Meisen, Street-Walters, Coppey) each induce a notion of exactness, and reciprocally, every notion of exactness determines a congruence on a bicategory of spans, in a certain way representing it.