Size-linewidth relation (σ_υ versus R) using the MR sample. The dashed line represents Larson’s first relation (Larson 1981; Solomon et al. 1987). The different colours represent four cloud classes (Sect. 3); ring-like clouds are represented in blue, elongated clouds in red, concentrated clouds in green, and clumpy clouds in magenta. The contours include 68% of the data (1–sigma level) and the small circles represent centroids (means) of the distributions. The expected slope for the relation is 0.5 (dashed black line) (Larson 1981).