The goal of this section is to help guide the researcher through the various model options for mixing tracers with isoneutral diffusion and/or one of the various skew-diffusion processes. These schemes explicitly mix only tracers. These schemes should be enabled concurrently with one basic mixing option discussed in Chapters 33 and 32. This section describes only those schemes which are implemented with constant diffusivities. Generalizations to nonconstant diffusivities are described in Section 34.2.