The theory of weight and balance is extremely simple. It is that of the familiar lever that is in equilibrium or balance when it rests on the fulcrum in a level position. The influence of weight is directly dependent upon its distance from the fulcrum. To balance the lever the weight must be distributed so that the turning effect is the same on one side of the fulcrum as on the other. In general, a lighter weight far out on the lever has the same effect as a heavy weight near the fulcrum. The distance of any object from the fulcrum is called the lever arm. The lever arm multiplied by the weight of the object is its turning effect about the fulcrum. This turning effect is known as the moment.
 

Similarly, an aircraft is balanced if it remains level when suspended from an imaginary point. This point is the location of its ideal CG. An aircraft in balance does not have to be perfectly level, but it must be reasonably close to it. Obtaining this balance is simply a matter of placing loads so that the average arm of the loaded aircraft falls within the CG range. The exact location of the range is specified for each type of airplane. {See unreferenced figure 3-1. An airplane suspended from its center of gravity (CG).