Consider an array of parallel perfectly-conducting wires extending between and normal to two infinite parallel perfectly-conducting planes. One wire acts as a resonator, having natural frequencies at which it is an integral number of half-wavelengths long. One might at first think that n wires would have n times as many natural frequencies. It turns out, however, that the modes characteristic of the system of many wires are plane electromagnetic waves with the field normal to the wires. All the modes of the array have frequencies for which the wires are an integral number of half-wavelengths long. In circuit terms, the wires, which act as resonators, are uncoupled. The electric and magnetic couplings are equal and opposite.