The choice of the parameter ξ determines the choice of gauge. The R ξ Landau gauge is classically equivalent to Lorenz gauge: it is obtained in the limit ξ → 0 but postpones taking that limit until after the theory has been quantized. It improves the rigor of certain existence and equivalence proofs. Most quantum field theory computations are simplest in the Feynman–'t Hooft gauge, in which ξ = 1 ; a few are more tractable in other R ξ gauges, such as the Yennie gauge ξ = 3.