This refers to the lock component where the key is inserted. A typical door knob has one keyhole, therefore one cylinder. Deadbolt locks are available in single-cylinder models and double-cylinder models. A double-cylinder model is operated from both sides with key, while a single-cylinder model uses a key from the outside and has a thumb-turn knob on the inside. Labor charges for rekeying locks is generally per cylinder. Therefore, if a door has a locking door knob and a double-cylinder deadbolt, there are 3 cylinders for the locksmith to service. If you are scheduling rekeying service, it is very helpful to have an accurate cylinder count. This allows us to provide an accurate cost estimate and to schedule ample time for the technician to complete the project.