In [[EM.Cube]]'s [[FDTD Module]], a physical structure consists of one or several objects that are grouped and identified by their material types. Materials are divided into five categories that are listed under the '''Physical Structure''' node at the top of the Navigation Tree:
* [[#Perfect Conductors|Perfect Electric Conductor (PEC)Objects]]* [[#Perfect Conductors|Perfect Magnetic Conductor (PMC)Planes]]* [[#Dielectric Materials|DielectricMaterials]]* [[#Anisotropic Materials|AnisotropicMaterials]]* [[#Dispersive Materials|DispersiveMaterials]]* Inhomogeneous Materials* Thin Wires
Under each material node, you can create new material groups of the same type/category but with different properties (color, texture, or electric and magnetic constitutive [[parameters]]). These material groups are used to organize the CAD objects that you draw in the project workspace or import from external model files. When you create a new geometrical object such as a Box or a Sphere, it is inserted under the currently active material type. There is only one material group that is active at any time. It is normally recommended that you first create material groups, and then draw new objects as part of the currently active material group. However, if you start a new [[FDTD Module]] project from scratch, and you start drawing a new object without having previously defined any material groups, a new default PEC group is created and added to the Navigation Tree to hold your new CAD object.