  Mobile holes in an antiferromagnetic insulator form a slowly fluctuating
array of quasi one-dimensional metallic stripes, which induce a spin gap or
pseudogap in the intervening Mott-insulating regions. The mobile holes on an
individual stripe acquire a spin gap via pair hopping between the stripe and
its environment; i.e. via a magnetic analog of the usual superconducting
proximity effect. This process is the analog of pairing in conventional
superconductors. At non-vanishing stripe densities, Josephson coupling between
stripes produces a dimensional crossover to a state with long-range
superconducting phase coherence. In contrast to conventional superconductors,
the superconducting state is characterised by a high density of (spin) pairs,
but the phase stiffness, which is determined by the density and mobility of
holes on the stripes, is very low.
