Silicon-on-insulator is a key technology to support the continuation of Moore's law. This publication investigates the impact of orientation, confinement, and strain on the electronic structure of thin silicon slabs using density functional theory. The comparative study of low-index orientations demonstrates that confinement not only widens the band gap but also transforms the band gap type. For thin silicon layers, strain can alter band gap and band gap type, too. By comparing our findings for different crystal orientations, we demonstrate that the consideration of the electronic structure of strained and confined silicon is of high relevance for modelling actual devices.