Constitutional rigidity and constitutional stability are different concepts. Rigid constitutions may be unstable and flexible constitutions may be more stable than rigid ones. This means that the stability of any constitution depends not so much on its form as on the "forces" and "principles" that stand behind it and support it. In post-war constitutions, fundamental principles shall not be subject to constitutional amendment. Also, Art. 138 of the Italian Constitution laid down a regulative principle. For this reason it not may be derogated or modified. This is not an obstacle to constitutional amendment though. Experience confirms that the constitutional reforms adopted in Italy have not required any change in Art. 138 of the Constitution.