U(1) is an Abelian group, meaning that the result is the same regardless of the order in which its elements are applied. QFTs can also be built on non-Abelian groups, giving rise to non-Abelian gauge theories (also known as Yang–Mills theories). Quantum chromodynamics, which describes the strong interaction, is a non-Abelian gauge theory with an SU(3) gauge symmetry. It contains three Dirac fields ψ, i = 1,2,3 representing quark fields as well as eight vector fields A, a = 1,...,8 representing gluon fields, which are the SU(3) gauge bosons. The QCD Lagrangian density is: