The Yakima SKS Lock Cores 4-Pack allows us to lock our Yakima racks and accessories with a single key for maximum convenience. We simply pop out the pre-existing lock core that came with our Yakima rack and install these interchangeable cores. This four-pack of SKS cores comes with two master keys, just in case we happen to misplace or lose one. It also includes a control key that removes existing unlocked cores, so we can swap in new locking cores.