CO column densities, N(CO), vs. OH column densities, N(OH) for three categories of clouds. Molecular clouds (MG) with CO detections are represented with black filled circles in the light yellow region. DMG-threshold clouds are represented by gray filled circles in the dark yellow region. In these clouds, CO emission is detected at the CO sensitivity of this work but would be detected at less than 3σ under a CO sensitivity of 0.25 K per 0.65 km s−1, typical of the CFA CO survey (Dame et al. 2001). The black triangles in the blue region represent DMG clouds in which CO emission is not detected at the CO sensitivity of the present work. The red solid line represents the least-squares fit result to the MG and DMG-threshold clouds.