Single cylinder deadbolts and handlesets are operated with a key from the outside, but do not require a key for locking or unlocking from the interior of the property. Instead, the door is locked or unlocked using a thumb-turn style lever.
Double Cylinder Deabolts
Double cylinder deadbolts or handlesets require a key for locking and unlocking from both the exterior and the interior of the property. There is no thumb-turn mechanism on a double cylinder lock.