A two layer printed circuit board is referred to as double sided board. Circuit boards with more than two layers are multilayer boards. There are many reasons a board design may require more than two layers. Some common reasons for the additional layers are, circuit routing density exceeds two layers, electrical properties, shielding or power distribution. The additional layers are referred to as inner layers. An inner layer is a thin dielectric (0.004” – 0.039”) with a copper pattern etched on each side. A print and etched inner layer is the most common type of inner layer. Inner layers and copper foil are bonded together with prepreg. Prepreg is the basic material from which all epoxy fiberglass circuit boards and laminate are made. Prepreg is fiberglass cloth pre-impregnated with a resin cured to a B Stage. A basic inner layer could have a power or ground on one side and signal traces on the other side. If two inner layers and 2 foil sheets were bonded together this would yield a six layer board. Four signal layers, one power and one ground layer.