Speaker wire is used to make the electrical connection between loudspeakers and audio amplifiers. Modern speaker wire consists of two electrical conductors individually insulated by plastic. The two wires are electrically identical, but are marked (e.g. by a ridge on the insulation of one wire, the color of one wire, a thread in one wire, etc) to help easily identify the correct polarity.
Some historic designs also featured another pair of wires for electrical power for an electromagnet in the loudspeaker. At least one such speaker design is still in production (in France), but essentially all speakers manufactured now use permanent magnets, which displaced field electromagnet speakers over half a century ago.
The effect of speaker wire upon the signal it carries has been a much-debated topic in the audiophile and high fidelity worlds. The accuracy of many advertising claims on these points has also been a matter of much debate.