Fractional differences in sound speed between our test suite solar models and solar data from S. Basu et al. (2009). The top panel shows differences between models with the same mixture that include different physical processes. Differences in nuclear reaction rates and surface boundary conditions are compared in the second panel. The third panel shows the impact of changes in the heavy element mixture; all of these models include diffusion but not rotation. The impact of different initial rotation rates and the transport model are compared in the bottom panel.