  If the standard model of quarks and leptons is extended to include three
singlet right-handed neutrinos, then the resulting fermion structure admits an
infinite number of anomaly-free solutions with just one simple constraint.
Well-known examples satisfying this constraint are $B-L$, $L_{\mu}-L_{\tau}$ ,
$B-3L_{\tau}$ , etc. We derive this simple constraint, and discuss two new
examples which offer some insights to the structure of mixing among quark and
lepton families, together with their possible verification at the Large Hadron
Collider.
