We plot line-widths (FWHM) of the Cygnus-X MDCs (red large dots and blue crosses) versus the core size taken from Motte et al. (2007). Black symbols indicate line-widths taken from the literature (Caselli & Myers 1995). Black line indicates a Larson like line-widths size-relation, while the dashed-dotted line is adopted from Plume et al. (1997) representing their best robust fit to their observed line-width size-relation of IRDCs. Gray line indicates the fiducial model of McKee & Tan (2003) with a surface density of ~1.7 g cm^-2 and a final stellar mass of 10 to 60 M_⊙. Dashed gray line indicates the same model with a clump surface density 3 times lower. A small box indicates the data presented in Fig. 10.