Vector addition of two or more force vectors can be done using the parallelogram law to obtain the resultant force. Consider a ship being pulled by two small tugboats. The two forces, F1 and F2, act on the ship concurrently. To find the net force using the parallelogram law, draw parallel lines to both the force vectors such that a parallelogram is formed. Now, the diagonal of the parallelogram represents the net force FN on the ship due to the two tugboats. It is expressed as the vector sum of forces F1 and F2 . Suppose now a third force from another tugboat acts on the ship. Another parallelogram is formed, using FN and the third force F3. The diagonal of this new parallelogram gives the resultant force on the ship due to all three tugboats. Here the resultant force on the ship is expressed as the vector sum of all three forces.