Any of several narrow regions within the main asteroid belt where few bodies are found, as a result of gravitational perturbations by Jupiter. These gaps occur at mean distances from the Sun which correspond to zones of commensurability. In these regions of the belt, an asteroid orbits the Sun in a simple fraction of Jupiter’s orbital period. These commensurabilities are generally written in the form of a ratio, for example 3 : 2 for objects which orbit in exactly two-thirds of Jupiter’s orbital period. A few zones of commensurability contain isolated concentrations of asteroids in stable orbits, but most are virtually devoid of asteroids. Examples are the gaps at the 2 : 1, 5 : 2, and 3 : 1 commensurabilities (mean distances of 3.28, 2.82, and 2.50 au from the Sun). Such gaps were first noted by D. Kirkwood in 1857.