What are single cylinder deadbolt locks?

A single cylinder deadbolt lock is a lock opened with a key from outside the house or apartment. A key isn't needed on the inside - the door is locked from the inside by turning a knob.

A double cylinder deadbolt needs to be opened with a key from both the inside and outside.

So a single cylinder deadbolt is considered the safest, since you don't have to search for a key when escaping from a fire or another emergency.

However, a single cylinder lock makes it easier for a burglar. If there is a glass panel in the door or a window close by, the burglar can break the glass, and reach in and open the door by turning the knob.

Some building codes prohibit double cylinder deadbolts for safety reasons.