where F is a differentiable vector field defined on a surface Σ which has a boundary curve C, that is, that the flux integral of the curl of F over the surface is equal to the work integral of F around the boundary of the surface. The surface and bounding curve need to be compatibly oriented; this means that when travelling along the boundary in the direction of the tangent dr and standing in the direction of the unit normal n the surface Σ is on your left.