(2) Here, R is the radius of curvature and  is the curvature. A curve with zero curvature is a straight line. The sign of  determines which way the helix turns with positive curvature being a right-handed (P ) helix and negative curvature a left-handed (M ) helix. Intuitively, curvature can be understood as to how big a cylinder the helix encloses: large curvature translates to a cylinder with a small radius. The torsion of the helix, ⌧ , is defined as