Crystals with a helical twist are not uncommon.
Helical motion and screws adapted to it are said to be right- or left-handed according to the G appearance presented by the rotation to an observer looking towards the direction of the translation.
The pitch of a screw is the distance, measured parallel to its axis, between two successive turns of the same thread or helical projection.
In wheelwork of this kind the contact of each pair of teeth commences at the foremost end of the helical front, and terminates at the aftermost end; and the helix is of such a pitch that the contact of one pair of teeth shall not terminate until that of the next pair has commenced.
He also invented, with the same object, teeth whose fronts, instead of being parallel to the line of contact of the pitch-circles, cross it obliquely, so as to be of a screw-like or helical form.