The single framework rule says that a specific quantum probabilistic model employs just one sample space and its associated event algebra, and in particular two incompatible frameworks cannot be combined. This rule, together with its extension to quantum dynamics, see subsection 4.2, is a central principle of histories quantum mechanics. It provides a guide for quantum reasoning that prevents one from falling into paradoxes; conversely, most quantum paradoxes are constructed by violating the single framework rule in some way.