[[Image:MAT1.png|thumb|400px|A structure made up of a PEC plate and different dielectric materials.]]
A physical structure in [[EM.Cube]] in made up of a number of geometric objects you either draw in the project workspace or import from an external CAD model file. In [[CubeCAD]], geometric objects are simply grouped together by their color. They do not have any physical properties. However, in all of [[EM.Cube]]'s computational modules, you need to assign physical properties to each geometric object. In these modules, geometric objects are grouped together by their common physical propertiesas well as their color. The types of physical properties may differ in different computational modules, but they are typically the same as related to the material properties or related boundary conditions.
From an electromagnetic modeling point of view, materials are categorized by the constitutive relations or boundary conditions that relate electric and magnetic fields. In general, an isotropic material medium is macroscopically characterized by four constitutive parameters: