A polynomial approximation of the flow surface of single crystals deforming plastically on various slip systems is considered. Because it involves an exponent n, it is known as the power law. It is shown that, by introducing deviations from the Schmid law (but deviations not discernible by measurement), an extension of the normality rule can be proposed: not only the flow direction, but also the rate of rotation due to the glide on the slip systems are obtained in a single algebraic formula. The validity of all this is assessed by comparing its predictions with the results of the. compression of single crystals in channel-die. The lateral reactions and the lattice rotations have been measured in typical rolling orientations and found in good agreement with the calculations.