It has been shown that the choice of renormalization scheme is crucial for four-quark operators, in particular for neutral kaon mixing beyond the standard model. In the context of SMOM schemes the choice of projector is not unique, it is part of the definition of the renormalisation scheme. I will present the non-diagonal Fierz relations which relate some projectors which differ by their Dirac-color structures.

Standard Model Parameters and Renormalization