  The ground-state properties of the square-lattice spin-1/2 Heisenberg
antiferromagnet with spatially anisotropic couplings are investigated by
Green's-function projection approaches. The staggered magnetization and the
two-spin correlators are calculated; the competition between magnetic long- and
short-range order is discussed in comparison with experiments on $\rm
Sr_2[Ca_2]CuO_3$.
