Time evolution of free-falling filaments in the Z − ∇V parameter space. All filaments have the same initial density n₀ = 4 × 10⁴ cm^-3. Each solid line represents the evolution of a filament with a different initial aspect ratio A₀ indicated at the bottom end. Dashed lines represent isochrones from 0.5 Myr to 4 Myr separated by 0.5 Myr. The solid and empty squares mark the positions of the four filaments, for projection angles of 45° and 67°, respectively (cf. text and Appendix A).