The system with an equivalent resultant force and a resultant couple moment having perpendicular lines of action can be reduced to an equivalent single resultant force acting along a new line of action. A concurrent force system has the lines of action of all the forces intersecting at a common point, so no moment is produced at this point and can be represented by a single resultant force. In a coplanar force system, the lines of action of all the forces and the resultant force lie in the same plane. So, the resultant couple moment is perpendicular to the resultant force. The resultant moment can be replaced by moving the resultant force by a perpendicular distance such that it produces the same moment. Similarly, in a parallel force system, all the forces and the resultant force are parallel to the same axis. And the resultant couple moment lies perpendicular to the resultant force. So, the system can be reduced to an equivalent single resultant force acting on the point lying on the perpendicular axis.