Steady laminar natural convection heal transfer in a two-dimensional, partially or fully open square cavity with three equally heated walls and various inclination angles is analyzed numerically. The overall average Nusselt number for a fully open cavity is approximately half the previously obtained average Nusselt number for a cavity with a heated back wall and insulated top and bottom walls for the same Rayleigh number. The dimensionless flow rale through the cavity is about twice the flow rate through the cavity with a healed back wall and two insulated walls. The effects of an aperture and cavity orientation, with respect to the direction of gravity, on natural convection heat transfer in the cavity are clarified.