A couple is a pair of parallel forces having equal magnitudes but acting in opposite directions, separated by a perpendicular distance. The couple causes a rotation force that rotates the body about an axis perpendicular to the plane of the forces, and the resulting moment is known as the couple moment. Consider tightening a bolt with a lug wrench. A force couple is formed when two equal and opposite forces are applied on the wrench at points A and B, separated by a perpendicular distance. The position vectors rA and rB are directed from the origin of a coordinate system to points of application. The couple moment about the origin is the sum of the cross-product of the position vectors and the forces. The difference between position vectors rA and rB is the position vector r, which is the distance between the two opposing forces. The couple moment only depends on the position vector r between the forces and not on the position vectors rA and rB. So, it is considered a free vector.