Screw operations are given the symbol R , where R and i are numbers greater than or equal to 1. Examples of symbols for screw operations are 2 or 4 (pronounced “two sub one” and “four sub three,” respectively). The operation involves choosing a vector→U and acts by a rotation of θ = 360/R degrees about an axis parallel to →U, together with a translation by i/R times →U; the translation direction relative to the rotation follows the right-hand rule. This rather complicated definition is, however, simple to visualize: it describes movement along a “spiral” (actually, helical!) staircase.