For first-order weak decays, that is processes involving only one quark decay, these quantum numbers (e.g. charm) can only vary by 1, that is, for a decay involving a charmed quark or antiquark either as the incident particle or as a decay byproduct, Δ C = ±1 ; likewise, for a decay involving a bottom quark or antiquark Δ B′ = ±1 . Since first-order processes are more common than second-order processes (involving two quark decays), this can be used as an approximate " selection rule " for weak decays.