Single cylinder locks are more common than double, though that trend is starting to change. Single cylinder locks have only one lock cylinder, as the name implies. Doors outfitted with single cylinders are unlocked with a key from the outside, but do not require a key for unlocking from the interior of the property. Instead, the door is unlocked with a thumbturn-style lever.
Double cylinder locks contain two cylinders. A key is required 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.
As already mentioned, single cylinder locks are more common than double cylinder. The two main reasons for this are efficiency and economy. Single cylinder locks are usually less costly to purchase and install than double cylinders. They are also more convenient, since there’s no need to hunt down a key to lock the door from the inside. A simple thumb turn locks the door.