A non-discrete screw axis isometry group contains all combinations of a rotation about some axis and a proportional translation along the axis (in rifling, the constant of proportionality is called the twist rate); in general this is combined with k -fold rotational isometries about the same axis (k ≥ 1); the set of images of a point under the isometries is a k -fold helix ; in addition there may be a 2-fold rotation about a perpendicularly intersecting axis, and hence a k -fold helix of such axes.