Snow chains, or tire chains, are evices which are affixed to the tires of vehicles to provide superior traction when driving through snow and ice. Snow chains are usually attached to the drive wheels of a vehicle (e.g. the front wheels on a front-wheel drive car), though all four wheels should be chained to provide extra stability. Some owner's manuals for four-wheel drive cars specify that two specific wheels can be chained, leaving the other two unchained if necessary.