As with any flavour-related quantum numbers, charm is preserved under strong and electromagnetic interaction, but not under weak interaction (see CKM matrix). For first-order weak decays, that is processes involving only one quark decay, charm can only vary by 1 (ΔC = ±1,0). 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.