87 Moreover, just as inhabitants of a town are wont to found associations with the widest diversity of purposes, which each is quite free to join or not, so those engaged in the same industry or profession will combine with one another into associations equally free for purposes connected in some manner with the pursuit of the calling itself. Since these free associations are clearly and lucidly explained by Our Predecessor of illustrious memory, We consider it enough to emphasize this one point: People are quite free not only to found such associations, which are a matter of private order and private right, but also in respect to them "freely to adopt the organization and the rules which they judge most appropriate to achieve their purpose." The same freedom must be asserted for founding associations that go beyond the boundaries of individual callings. And may these free organizations, now flourishing and rejoicing in their salutary fruits, set before themselves the task of preparing the way, in conformity with the mind of Christian social teaching, for those larger and more important guilds, Industries and Professions, which We mentioned before, and make every possible effort to bring them to realization.
Source: Quadragesimo Anno (Vatican.va)