A diagram that represents the different types of chemical bonding in materials (particularly solids). The three vertices of the triangle are covalent, metallic, and ionic, with materials being either on the edges of the triangle or inside it. This representation was used first by the Dutch chemist Anton Eduard van Arkel in 1941 and then by the Dutch chemist Jan Arnold Albert Ketelaar in 1947. It has been used very widely since the 1950s, with many extensions and elaborations of the original work of van Arkel and Ketelaar.