Art. 39. Trade unions may be freely established. No obligations may be imposed on trade unions other than registration at local or central offices, according to the provisions of the law. A condition for registration is that the statutes of the trade unions establish their internal organisation on a democratic basis. Registered trade unions are legal persons. They may, through a unified representation that is proportional to their membership, enter into collective labour agreements that have a mandatory effect for all persons belonging to the categories referred to in the agreement. Art. 40. The right to strike shall be exercised in compliance with the law.