This presentation looks into the universe of Knowledge Organisation Systems (of well KOS). KOS are made for ordering knowledge and aspire generic validity and stability over time. But, KOS also evolve with the body of knowledge for which they are designed. In general, KOS are complex evolving systems. As bodies of knowledge grow, change and new ones emerge, the need also grows to create layers to connect them. In short, KOS live in the tension between supporting interoperability while serving specific needs. In this presentation we look into the history of KOS (in the case of the Universal Decimal Categorization, the case of WikiCategories) and recent requirements coming from movements as FAIR.