Their helix was right-handed, meaning its spiral turns the same way as a normal, right-handed screw.

Its mirror image with the right-handed screw axis is given the Greek symbol Δ (delta).

When the lines are skew, the sign of the result indicates the sense of crossing: positive if a right-handed screw takes L into L', else negative.

In an optically active medium, the polarization direction twists in the same sense (e.g. like a right-handed screw) during the forward and backward passes, whereas in a Faraday medium, because the light reverses its propagation direction with respect to the magnetic field, the helicity of the propagation also reverses.