The current assumption of exclusivity as a basic characteristic of the civil law concept of ownership, and as the main reason for the civil law aversion towards a fragmentation of ownership, needs to be revised. In this doctoral thesis different exclusivity theories have been discovered from which a double dimension of exclusivity can be derived. The external exclusivity, the owner’s right to exclude others, is a constant principle in our historical and comparative research. The internal exclusivity, the full amount of property rights on a certain good at the same time, is a typical characteristic of the modern civil law ownership theory. An owner retains the prospect of becoming the full owner, even when he grants far-reaching rights to third parties, because of the elasticity or residuarity of ownership. The internal exclusivity remains the general principle, but exceptions are possible. The (legal, contractual and jurisprudential) modulation of the exclusivity principle and the study of fiduciary ownership and co-ownership illustrate the need for a functional approach. The (external) legal outcome of legal instruments matters most, despite the question who is the (internal) exclusive owner. A split of ownership is also possible in civil law legal systems. Recent developments in property law, insolvency law and patrimonial family law show that there is a tendency to create layers of ownership. However, the theory of internal exclusive ownership also reveals that the alleged split of ownership created by a trust is false. A new model for the civil law trust is presented which focuses on the split of patrimony as an alternative to the split of ownership.