Insulators are usually made of porcelain, a ceramic material which does not conduct electricity. They connect the wires to the poles or cross-arms so that electricity will not flow from wire to wire or down the poles to ground. Some styles of insulators require a cross arm to support the insulator while other styles have fittings and hardware that allow the insulator to be supported directly off the main pole. Construction practices vary from utility to utility.
Electrical codes dictate the minimum distance a given voltage on a circuit may be to the cross-arm or the pole. The higher the voltage, the larger the distance required. It is not usually necessary to "stack" insulators together for distribution system voltages like they commonly are on the transmission system. Various sizes of insulators are readily available for common distribution voltages.