The previous discussion is now specialized to particular forms of mixing used in MOM. The first form is Cartesian or horizontal-vertical diffusion in which the principle axes are assumed to be defined by the local tangent to the geopotential surface; i.e., the constant Eulerian (x,y,z) frame (the term z-level is used in the subsequent). The tracer mixing tensor is diagonal in the (x,y,z) coordinate system and there is no anti-symmetric component. Additionally, the components of the diffusion tensor in the horizontal directions are generally taken to be roughly 107 times larger than the vertical components. This large anisotropy arises from the strong vertical stratification in most regions of the ocean which suppresses vertical mixing.