First, let's define a cylinder. The lock cylinder is not the whole lock. It is only the part of your lock mechanism that the key is inserted into. Most locks have removable and rekeyable cylinders. This allows you to change the key that works with the lock without going out and buying a new lock.
A Euro, CES or profile cylinder (CES is a brand of cylinders used by Hoppe, a very common brand) is shaped like an enlarged key hole. It is put through the lock case after it has been mortised into the edge of a door.
These Euro cylinders come as double cylinders, which are keyed on both sides, single cylinders which are keyed on one side and dummy cylinders, which have no key and a thumbturn on one side.
The size of profile cylinder you need is determined by the thickness of the door and the depth of the back plates on your lock set. Adding these values together will give you the correct length of your CES cylinder. Common sizes are 70mm, 64mm and 120mm.
Profile cylinders are also classed as 90 degree cylinders or 360 degree cylinders. A 90 degree cylinder works like a normal lock cylinder. Turn the key or thumbturn one quarter of a turn and your door is locked or unlocked. The 360 degree cylinder must be turned a full 360 degrees to lock or unlock.
Look for Euro cylinders to become more common in the United States in the future, especially as more high security multi-point locks are being installed in new homes. Multipoint locks use profile cylinders.
