Comparison of the spectral shape of the (1 − 0) (top) to (4 − 3) (bottom) lines of ¹³CO for two different combinations of kinetic temperature, volume density, and column density. The plain blue and dashed red histograms show the spectra for (22 K, 10⁵ cm⁻³,10¹⁸ cm⁻²) and (60 K, 10^1.65 cm⁻³, 10^17.9 cm⁻²), respectively. The lines share the same centroid velocity C_V = 0 km s⁻¹ and velocity dispersion σ_V = 0.32 km s⁻¹. The two sets of physical conditions deliver degenerate spectral profiles within the noise level for the (1 − 0) and (2 − 1) lines. Additional information, e.g., the (3 − 2) or (4 − 3) lines, is needed to lift this degeneracy.