In this work the experimental verification of the appearance of square patterns in Rayleigh–Be´nard systems is described. Convective square cells are observed above the threshold of convection in a layer of water sandwiched between two Plexiglas plates. These experimental results agree with earlier theoretical predictions [J. Fluid Mech.139, 461 (1984);152, 113 (1985);178, 491 (1987)], and provide, as far as can be determined, the very first direct observation of the stability of square patterns due to a hybrid thermal boundary condition on the top and bottom of the convecting fluid layer.