In general the starting point of the definition and existence theorem of an invariant is a presentation of the objects in the class under consideration, and a calculus for this presentation. More precisely one has a class of concrete objects, a rule to associate a 3-manifold to each object, and a set of moves such that if two objects define the same 3-manifold then they are obtained from each other by a finite number of these moves. In this work I provide a calculus for compact 3-manifolds with boundary based on standard spines, and discuss related questions; in particular, I introduce a general framework for the definition of invariants based on this calculus.