On the most basic PCB, the parts are concentrated on one side and the wires on the other. Because wires appear on only one side, this type of PCB is called Single-sided. Only early circuits used such boards because a single panel had many severe limitations on the design circuit (because there was only one side, the wiring could not be crossed and had to be routed in a separate path). Multilayer, multilayer wire, must have the appropriate circuit connection between the two sides. This "bridge" between circuits is called a pilot hole (via). A pilot hole is a small hole in the PCB, filled or coated with metal, that can be connected to the wires in each layer. This allows multiple components to be connected