B-rep for CAD. Boundary representation is essentially a local representation connecting faces, edges and vertices. An extension of this was to group sub-elements of the shape into logical units called geometric features, or simply features. Pioneering work was done by Kyprianou in Cambridge also using the BUILD system and continued and extended by Jared and others. Features are the basis of many other developments, allowing high-level "geometric reasoning" about shape for comparison, process-planning, manufacturing, etc. Boundary representation has also been extended to allow special, non-solid model types called non-manifold models. As described by Braid, normal solids found in nature have the property that, at every point on the boundary, a small enough sphere around the point is divided into two pieces, one inside and one outside the object. Non-manifold models break this rule. An important sub-class of non-manifold models are sheet objects which are used to represent thin-plate objects and integrate surface modelling into a solid modelling environment. Standardization Standardization for boundary representation took time to develop. In a meeting organized by the Computer-Aided Manufacturing International (CAM-I) in 1979 the IGES format was discussed for solid model transfer. IGES was not, then, suitable. Another complication was the coexistence of, then, two major representations, CSG and Boundary Representation, although use of CSG in commercial systems started to decline later. Further developments within CAM-I led to the Experimental Boundary Format, known as XBF, which was proposed to IGES as a possibility for extension to cover Boundary Representation models. However, this was not taken up. Towards the end of the 1980s a project called CAD*I developed a standard representation which then became one of the bases for the development of the STEP solid model format, the first widely accepted data exchange format for Boundary Representation. In the world of data-exchange, STEP, the Standard for the Exchange of Product Model data also defines some data models for boundary representations in a neutral form which can be mapped to specific data structures. The common generic topological and geometric models are defined in ISO 10303-42 Geometric and topological representation. The following Application Integrated Resources (AICs) define boundary models that are constraints of the generic geometric and topological capabilities: ISO 10303-511 Topologically bounded surface, definition of an advanced face, that is a bounded surface where the surface is of type elementary (plane, cylindrical, conical, spherical or toroidal), or a swept surface, or b spline surface. The boundaries