With so many Door Deadbolts it can often be difficult to figure out which one is best for your application. With over 40 years experience our experts at WindowDoorParts can help you decide. There are two different kinds of deadbolt lock, Single Cylinder and Double Cylinder. Single cylinder deadbolts are operated with a key from the outside but do not require a key for locking inside the door as they are locked or unlocked with a thumb-turn type of lever. A double cylinder lock there is no thumb-turn mechanism.