Mach number of the vertical gas velocity as a functionof radius r and height z in a simulation including a locally isothermal gas, but no particles (iso_Lr=90au_Lz=4Hg). The simulation domain spans four gas scale heights in the vertical dimension. Black lines mark one gas scale height above and below the midplane. After 30 kyr, the vertical shear instability has saturated at all radii and heights; it gives rise to perturbations which are characterized by a radial-to-vertical wavelength ratio much smaller than unity, by being bent outwards, and by a mirror symmetry relative to the midplane.