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**Charm** (symbol * C*) is a flavour quantum number representing the difference between the number of charm quarks (

By convention, the sign of flavour quantum numbers agree with the sign of the electric charge carried by the quarks of corresponding flavour. The charm quark, which carries an electric charge (*Q*) of +2⁄3, therefore carries a charm of +1. The charm antiquarks have the opposite charge (*Q* = −2⁄3), and flavour quantum numbers (*C* = −1).

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.

- Lessons in Particle Physics Luis Anchordoqui and Francis Halzen, University of Wisconsin, 18th Dec. 2009