Page "Curl (mathematics)" Paragraph 10

where is a line integral along the boundary of the area in question, and | A | is the magnitude of the area.

If is an outward pointing in-plane normal, whereas is the unit vector perpendicular to the plane ( see caption at right ), then the orientation of C is chosen so that a tangent vector to C is positively oriented if and only if forms a positively oriented basis for R < sup > 3 </ sup > ( right-hand rule ).