The orientation and disorientation of extended cell block boundaries that separate cell blocks in model rate-independent grains accommodating imposed plastic deformation by the mechanism of slip is predicted on the basis of the following three hypotheses: (1) a uniform state of stress prevails throughout the grain; (2) cell blocks are disoriented so as to minimize the power of plastic deformation; and (3) cell block boundaries are oriented so as to minimize plastic incompatibility between neighboring cell blocks. Predicted orientations and disorientations compare favorably with those reported in the experimental literature for copper and aluminum polycrystals deformed plastically in uniaxial tension. This suggests that the assumed hypotheses may represent the physical principles that determine the preferred cell block boundary orientation.