A circle with radius has a constant curvature of

whereas a line has a curvature of 0.

Binormal vector

The unit binormal vector is the third Frenet vector . It is always orthogonal to the unit tangent and normal vectors at . It is defined as

In 3-dimensional space, the equation simplifies to

or to

That either sign may occur is illustrated by the examples of a right-handed helix and a left-handed helix.

Torsion

The second generalized curvature is called torsion and measures