Brushes are used in some 3D video games such as games based on the Source engine, its predecessor the Goldsrc engine, Unreal Engine's tool Unreal Editor, etc. to construct levels.[1] Brushes can be primitive shapes (such as cubes, spheres & cones), pre-defined shapes (such as staircases), or custom shapes (such as prisms and other polyhedra). Using CSG operations, complex rooms and objects can be created by adding, subtracting and intersecting brushes to and from one another.