The Open Vault is an attempt to examine and critically extend Robert Venturi’s idea of detached and semi-detached linings. Spatially, a semi-detached lining explores how partially detached inner and outer layers can create habitable space. Using Venturi’s idea as a point of departure, a working definition was developed: A semi-detached lining consists of single or multiple linings that maintain partial contact with each other to create interstitial space. Nested volumes and tangent surfaces are utilized in order to implement this spatial concept.