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Some lock hardware is designed to use interchangeable cores. While standard key-in-knob, mortise, and rim locks require some disassembly of the locks to get to the lock cylinder, interchangeable core locks allow the core to be removed from the lock with the use of a core key, also known as a control key. A standard lock cylinder appears to have a round core where the key is inserted. An interchangeable core appears more like a figure-8. The small format interchangeable core (SFIC) was made popular some time ago by the Best lock company. Some refer to them as Best or Best-style locks. More recently, other manufacturers have come up with their own interchangeable core designs, usually referred to as large format interchangeable cores (LFIC), but their cores are designed to be used with their hardware. There is a full range of locking hardware now made by several manufacturers using the earlier SFIC design. Regardless of which type, a critical part of servicing these locks is the availability of the core key. In many cases, with no core key, you may end up having to replace the whole lock rather than just the core. With the core key, servicing or changing the locks is a quick and simple matter.

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