In quantum chromodynamics (QCD), the quantity Λ is called the QCD scale. The value is Λ Λ M S = 332 ± ± 17 MeV for three "active" quark flavors, viz when the energy–momentum involved in the process allows production of only the up, down and strange quarks, but not the heavier quarks. This corresponds to energies below 1.275 GeV. At higher energy, Λ is smaller, e.g. Λ Λ M S = 210 ± ± 14 MeV above the bottom quark mass of about 5 GeV. The meaning of the minimal subtraction (MS) scheme scale Λ MS is given in the article on dimensional transmutation. The proton-to-electron mass ratio is primarily determined by the QCD scale.