On a typical door – either wood, metal, or plastic (yes, that’s not a typo, a lot of new doors are made of a plastic shell filled with foam) – there are two locks: a doorknob lock and a deadbolt. Deadbolts come in two types – single-sided (or “single cylinder”) and double-sided (“double cylinder”). A single-sided deadbolt uses a key on the external side of the door, and has a thumb-turn (no need for a key) on the internal side. Most fire codes recommend or require single-side