  Gaseous nuclear rings are large-scale coherent structures commonly found at
the centres of barred galaxies. We propose that they are an accumulation of gas
at the inner edge of an extensive gap that forms around the Inner Lindblad
Resonance (ILR). The gap initially opens because the bar potential excites
strong trailing waves near the ILR, which remove angular momentum from the gas
disc and transport the gas inwards. The gap then widens because the bar
potential continuously excites trailing waves at the inner edge of the gap,
which remove further angular momentum, moving the edge further inwards until it
stops at a distance of several wavelengths from the ILR. The gas accumulating
at the inner edge of the gap forms the nuclear ring. The speed at which the gap
edge moves and its final distance from the ILR strongly depend on the sound
speed, explaining the puzzling dependence of the nuclear ring radius on the
sound speed in simulations.
