Schematic view of the MC structure, illustrated with typical parameter values estimated using ¹²CO, ¹³CO, and C¹⁸O (J = 1–0) lines. The size of each parameter sign indicates the magnitude of the typical value. Color depth indicates H₂ number density (n), thick arrows indicate pressure (P), and circular arrows indicate turbulence (Δv). The actual ¹³CO- and C¹⁸O-bright regions are smaller than shown in the schematic, and some of them are clumpy. The typical area ratio of ¹³CO to ¹²CO is ∼0.26 for the overall MC sample. For MCs with significant C¹⁸O emission, the typical area ratio of ¹³CO to ¹²CO is ∼0.45, and the C¹⁸O to ¹²CO ratio is ∼0.06. The stratified structure of ¹²CO and ¹³CO remains valid for MCs in which no C¹⁸O emission is detected.