Best Keys

An interchangeable core system has two kinds of key doing two different jobs. The operating key turns the lock as any key would. The control key does not open the door at all, it retracts the lug that holds the core in place so the core can be pulled and replaced.

Losing track of which is which is the usual problem on these systems. Keep the control keys separate from the operating keys and mark them, because a control key is effectively the key to every lock the core fits.