What determines longitudinal static stability of an aircraft?

Longitudinal static stability is determined by the position of the center of gravity (CG) relative to the neutral point (NP). The neutral point is where adding lift causes no pitching moment change. If CG is forward of NP, the aircraft is statically stable - increasing angle of attack creates a nose-down restoring moment. The distance between CG and NP, expressed as a percentage of mean aerodynamic chord, is the static margin. Commercial aircraft typically have static margins of 10-15% for positive stability with acceptable control forces.