A proposal for a presentation given at the Open Repositories conference in 2015 held in Indianapolis described thus, One of the many successes of the Hydra community is the fundamental notion from which its name is derived—the concept of many interfaces (“heads”) over top of a single repository (the “body”). The recent release of Fedora 4, with its internal RDF-centric model, has spurred efforts for a community-wide model of collections and works, such that the heads can be sure that the body will behave as they expect it to. That model has been designed and vetted by the Hydra community, and its architecture and initial implementations will be presented in this paper. [Note, and the subject of this proposal has since become known as the 'Portland Common Data Model'.]