J is one finger direction of the matrix (up, right, forward). Although many people believe mathematics should use the right hand rule ( http://en.wikipedia.org/wiki/Right-hand_rule ), I think it's better to use the left hand rule ( http://en.wikipedia.org/wiki/Left-hand_rule ), since all good things should grow in positive direction (up=good, right=good, forward=good). Also in 3D graphics, X+ means always right, Y+ means up, Z+ means forward, although many 3D programs do it wrong.