The chirality of a molecule that has a helical, propeller, or screw-shaped geometry is called helicity or helical chirality. The screw axis or the D, or C principle symmetry axis is considered to be the axis of chirality. Some sources consider helical chirality to be a type of axial chirality, and some do not. IUPAC does not refer to helicity as a type of axial chirality.
Enantiomers having helicity may labeled by using the prefix notation (P) ("plus") or Δ (from Latin dexter, "right") for a right-handed helix, and (M) ("minus") or Λ (Latin levo, "left") for a left-handed helix. The P/M or Δ/Λ terminology is used particularly for molecules that actually resemble a helix, such as the helicenes. This notation can also be applied to non-helical structures having axial chirality by considering the helical orientation of the Cahn–Ingold–Prelog group rankings of the "front" groups compared to the "back", when viewed from either direction along the axis.