When ordering an entryset, deadbolt, or sideplate set you will be asked for the function of the deadbolt. Your choices typically are single cylinder or double cylinder. Here’s how to decide which you should use.
The most common option is single cylinder. This means that the deadbolt has one cylinder with a key on the outside. On the inside there is a turn piece. To lock or unlock the door from the inside you pivot the turn piece. To lock or unlock the door from the outside, you insert the key and turn.
The other option is double cylinder. This lock has two cylinders, one for each side. Therefore to lock or unlock from either side you’ll need a key. Usually you’ll see a double cylinder on a door that has glass in the door, or next to the door. You wouldn’t want someone to break the glass and reach in to unlock the door.
Something to really give some thought on is if there is an emergency and you have to exit via only that door you won’t be able to without a key. You’ll also want to double check the building codes in your area to be sure that a double cylinder is allowable. A place you would normally see a double cylinder is on a storefront. These doors are typically unlocked during business hours and you wouldn’t want anyone other than an employee to be able to lock the door with the turn piece while the business is open.
A few deadbolts are also available as a function called single sided or thumbturn only. Basically what these are is a turn piece and latch bolt only. Typically you wouldn’t drill the hole all the way through the door and there would be no way to get in from the outside if the door is locked. You might see this option on a second story balcony or a delivery/receiving entrance to a building.
Now that you have the knowledge on the different functions and when to use them, you’ll be able to make the right choice for your situation.