Clumpy (upper panel; inspired by Fig. 4 in Wolfire et al. 1993) and tangled (lower panel) macrotubulent models. The sketches represent the corresponding line of sight (left) and projected (center) views of the cloud, as well as the expected line profiles in the case of optically thin emission (right; e.g., C¹⁸O observations). The broadest and highly supersonic linewidths have different origins in these two models. Owing to their large filling factor along the line of sight, they are generated by the accumulation of a high number of sonic-like clumps at different velocities in the classical clumpy models. On the contrary, in the proposed tangled clouds, the linewidths arise from the superposition of few transonic structures correlated at large scales, both spatially and in velocities (i.e., fibers).