39 Because sociability is one of man's natural requirements and since it is legitimate to promote by common effort decent livelihood, it is not possible without injustice to deny or to limit either to the producers or to the laboring and farming classes the free faculty of uniting in associations by means of which they may defend their proper rights and secure the betterment of the goods of soul and of body, as well as the honest comforts of life. But to unions of this kind, which in past centuries have procured immortal glory for Christianity and for the professions an untarnishable splendor, one can not everywhere impose an identical discipline and structure, which therefore can be varied to meet the different temperament of the people and the diverse circumstances of time.
Source: Sertum Laetitiae (Vatican.va)