All leptons carry a lepton number L = 1. In addition, leptons carry weak isospin, T 3, which is − ⁠ 1 / 2 ⁠ for the three charged leptons (i.e. electron, muon and tau) and + ⁠ 1 / 2 ⁠ for the three associated neutrinos. Each doublet of a charged lepton and a neutrino consisting of opposite T 3 are said to constitute one generation of leptons. In addition, one defines a quantum number called weak hypercharge, Y W, which is −1 for all left-handed leptons. Weak isospin and weak hypercharge are gauged in the Standard Model.