Values of the z/r–M_d relationship for various CO isotopes and transitions, assuming an ISM-like volatile carbon abundance. The left panel indicates results from the fiducial T Tauri models. The right panel the same but for the Herbig stellar parameters case. Some low-mass disk models are missing z/r values for ¹³CO and/or C¹⁸O due to their low abundance and lack of optically thick emission surface.