Quarks have color charge, so they have strong interactions. Strong interactions cause quarks to combine into hadrons. Residual strong interactions bind hadrons together to form a nucleus.

The different quark types (u, d, c, ...) are referred to as flavor. Flavor is altered only by charged weak interactions.

However, when a quark emits a (virtual) W⁺ or W^- boson, it must change its electric charge and thus, its flavor. The predominant weak processes involve transitions between quarks in the same generations, but rarer transitions also occur between any +2/3 quark and any -1/3 quark.

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The quark chart. The lepton chart.