A class which refers to another points to it with an arrow. (This does mean ListOfAtoms has pointers to a BoundaryCondition, a NeighborLocator, and a Constraint.) Derived classes point to their bases with hollow arrows. The numbers on either end of lines connecting classes indicate multiplicity. For instance, a ListOfAtoms has only one BoundaryCondition, but a BoundaryCondition can be associated with more than one list of atoms.