A capacitor basically consists of opposing electrically conducting plates separated from each other by a dielectric, or insulating substance. An electric field builds up between the two surfaces. The resistance of a capacitor is high for low frequencies but low for high frequencies. This makes it possible to use capacitors for separating low and high frequencies. The "size" of a capacitor is described as its "capacity". Depending on the dielectric used, we differentiate between electrolytic capacitors and metallised capacitors. Only bipolar radio frequency capacitors may be used for crossovers because electrolytic capacitors, as used in other applications, only conduct current in one direction.