A union catalog (abbreviation: VK ; synonyms : catalog association, association database ) is a joint library catalog of several libraries . The libraries participating in the union catalog are usually combined in a library association . The common catalog basically contains all media from all participating libraries. From a technical point of view, a union catalog is now implemented as a database that is filled with bibliographic data by the participant libraries. The servers on which the central database of the network is stored are usually located in the network headquarters or in one of the participant libraries.