Geometrical constraints in a molecule can cause a severe distortion of idealized tetrahedral geometry. In compounds featuring "inverted" tetrahedral geometry at a carbon atom, all four groups attached to this carbon are on one side of a plane. The carbon atom lies  at or near the apex of a square pyramid with the other four groups at the corners.
The simplest examples of organic molecules displaying inverted tetrahedral geometry are the smallest propellanes, such as [[1.1.1-Propellane|[1.1.1]propellane]]; or more generally the paddlanes, and pyramidane ([3.3.3.3]fenestrane). Such molecules are typically strained, resulting in increased reactivity.