Natural convection heat transfer from the face-to-face surfaces of parallel, square vertical plates was investigated experimentally. The experiments encompassed three types of hydrodynamic conditions along the lateral edges: (1) fully open to ambient, (2) blockage of one of the edge gaps, (3) blockage along both of the edge gaps. Measurements were made for ten interplate spacings. Use of a mass transfer measurement technique (the naphthalene sublimation technique) instead of direct heat transfer enabled elimination of radiation effects, extraneous convectiveI'conductive heat losses, and variable property effects. The results for the fully open-edged case cast substantive doubt on the oft-quoted data of Elenbaas in the low range of the parameter (b/H)Ra, where b is the interplate spacing and H is the plate height. Uncertainties in Elenbaas' results owing to large fluid property variations are also demonstrated (the present results correspond to constant properties). Suppression of fluid flow along one lateral edge gap generally has only a modest effect on the transfer coefficients and no effect for (b/H)Ra > 4. When both edges are blocked, 30 percent (or more) reductions in the coefficients occur at lower (b/H)Ra. When (b/H)Ra > 10, the lateral edge conditions do not affect the results.