Refer to figure.
Longitudinal stability is motion about the lateral axis.

Static Longitudinal stability is the aircraft's natural tendency when disturbed in pitch, to return to its former trimmed angle of attack without pilot input, and is desirable throughout the aircraft's complete speed range.

For the study of stability, we consider the changes in magnitude of lift force due to changes in angle of attack, acting through a stationary point, the aerodynamic centre (AC).

There are two moments to consider: the wing moment and the tail moment. The wing moment is a function of the change in wing lift multiplied by arm ‘x’. The tail moment is a function of the change in tailplane lift multiplied by arm ‘y’. The length of both arms is dependent upon CG position. If the CG is considered in a more forward position, the tail arm is larger and the wing arm is smaller. A more forward CG position increases static longitudinal stability.

If the nose-down (negative) tail moment is greater than the nose-up (positive) wing moment, the aircraft will have static longitudinal stability.