I have the type which comes with four bolts with special heads and two identical "keys" that you fit on the lug-wrench to twist on the bolts. This is supposed to be the strongest type (similar to Gorilla locks), since the thief cannot turn the bolt if he doesn't have the special attachment key for the lug-wrench. The other type, which consists of a lock over the bolt and a key which closes it, can be broken off with a crow bar.