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We have carried out a back-reflection x-ray standing-wave (BRXSW)investigation to study the adsorbate ordering in the system (1X2)Rb/Cu(110), which corresponds to an alkali-induced missing row reconstruction of the Cu(110)surface. By carrying out measurements in three different scattering geometries, we find that for the missing row structure at room temperature, the Rb adatoms are extremely well ordered in the direction perpendicular to the surface, show a high degree of ordering across the missing rows (e.g., the atoms are locked into the missing row troughs), but are highly disordered along the rows, similar to a one-dimensional lattice gas. We compare our results to the Rb/Cu(110)-saturated surface at room temperature and to our previous BRXSW investigation of Rb/Cu(111).