As was just pointed out, Roller wear is also a factor to be taken into account. Simply measuring 12" of a chain doesn't take roller wear into account. A local bike mechanic showed me a little trick. Put your chain on the big ring. Grasp the link that is at the front of the ring and pull it forward. According to him, if you can see more than half of the tooth that was behind that link, it is time to change the chain. The beauty of this test is that it takes stretch AND roller wear into account.