The rotation of galaxy disks influences ISM structures in several ways. Since the angular velocity declines with increasing distance from the centre, any ISM feature, such as giant molecular clouds or magnetic field lines, that extend across a range of radius are sheared by differential rotation, and so tend to become stretched out in the tangential direction; this tendency is opposed by interstellar turbulence (see below) which tends to randomize the structures. Spiral arms are due to perturbations in the disk orbits - essentially ripples in the disk, that cause orbits to alternately converge and diverge, compressing and then expanding the local ISM. The visible spiral arms are the regions of maximum density, and the compression often triggers star formation in molecular clouds, leading to an abundance of H II regions along the arms. Coriolis force also influences large ISM features.