Now consider the possibility of a moment about the X axis. Notice how the F x force is parallel to the X axis. The F x force will not cause any twisting or rotation about the X axis. Remember that moment equals force times eccentric distance. Note however, that the forces cause twisting in opposite directions about the X axis.
Some cancellation has to occur.
To account for this cancellation, we use the Right Hand Rule. Imagine a moment about the X axis. This moment is like the curving of the fingers when you form a fist with your rigth hand. The thumb points in the positive direction along the X axis. The moment can be visuallized as a double headed vector.