23First, the general point should be made that a contract is characterised by the principle of freedom of the parties to arrange their own affairs, according to which, in particular, parties are free to enter into obligations with each other. Under that principle, and in a situation such as the one in the main proceedings where the defendant is not a member of any employers' association and is not bound by any collective agreement, the rights and obligations arising from such an agreement do not therefore apply to it, as a rule. Otherwise, as the Advocate General noted in point 52 of his Opinion, the principle that contracts cannot impose obligations on third parties would be infringed.