A planar curve may be defined by use of intrinsic properties such as arc length s (from some fixed point of the curve) and the angle ψ‎ that the curve’s tangent makes (with a fixed direction). The curvature κ‎ then satisfies κ‎ = dψ‎/ds and alternatively κ‎ and s might be used to define the curve. For a curve in three dimensions, arc length, curvature, and torsion can be used.