Critical Rayleigh numbers, roll configurations, and growth-rate derivative are calculated at onset of convection for a rigid box with conducting upper and lower plates and insulating sidewalls. When the sidewalls form a square or a near square, the linearized Oberbeck–Boussinesq equations favour crossed rolls, a superposition of three-dimensional rolls in the x- and y-directions, over unidirectional rolls. These crossed rolls preserve the four-fold rotation symmetry about the vertical axis of a square box only when the aspect ratio (ratio of width to depth of the box) demands an even number of rolls in each direction. The analysis explains patterns observed by Stork & Müller.
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