The normal and shear strains obtained in torsion shear tests may be interpreted in two different ways to gain insight into (1) the type of plastic potential to be employed in hardening plasticity stress-strain models, and (2) the coincidence in physical space of the plastic strain increment direction with the stress direction during principal stress rotation. Thirty-four drained torsion shear tests were performed on hollow cylinder specimens of Santa Monica Beach sand deposited by dry pluviation. Twenty-six tests were performed on tall specimens with height of 40 cm, and eight tests were performed on short specimens with height of 25 cm to investigate the effect of the specimen height on the soil behavior in hollow cylinder specimens. Each test was conducted with the same, constant inside and outside confining pressure, r r , thus tying the value of b = (r 2 -r 3 )/(r 1r 3 ) to the inclination, b, of the major principal stress. The directions of strain increment vectors at failure are compared with the directions corresponding to associated and non-associated flow. The relation between the directions of major principal strain increment and major principal stress during rotation of principal stress axes in physical space are investigated.