This is a quick way of finding out if the forces acting on an object are in equilibrium.

This object experiences three forces.

If it is in equilibrium then drawing accurate vector diagrams of each force one after the other will produce a closed triangle.

In other words, the total amount of force pushing left = the total amount of force pushing right...

and...

the total amount of force pushing up = the total amount of force pushing down.

If the forces are not in equilibrium, the triangle will not close.

In order to get this system to equilibrium, you need to add an additional force that would connect the end of H with the start of G. This is shown by the dotted line.

If you use scale drawings you can measure the forces needed directly from the diagram. The example below takes you through this step by step: