Steady laminar natural convection of water about vertically stacked two-sided, horizontal heated plates was studied experimmentally over a range of plate gap to plate half-width ratio, H/a, from 0.078 to 0.94. Measurements were made of power input per plate, plate temperatures, and inlet and outlet bulk fluid temperatures. From these data were determined average Nusselt and Rayleigh numbers for each plate, which were correlated with a power law Nu = CRan for each gap height. The Nusselt numbers increased with gap height up to H/a = 0.47 (from 8 to 48), and then became independent of H/a. The exponent n increased from n similar or equal to 0.2 at the lowest gap height tested (H/a = 0.078) to a high value of n similar or equal to 0.4 at H/a = 0.24 and then decreased back to n = 0.2 for H/a greater than or equal to 0.47.