This is a common question that has some significant issues you will want to consider before purchasing a Dead Bolt. The most common dead bolt in use is the single cylinder dead bolt where a key is required to unlock the door from outside but a simple thumb-turn is used to unlock the door from inside.
A double cylinder dead bolt requires a key to unlock from outside or inside the house. Double cylinder dead bolts are considered more secure in doors with glass or doors with glass sidelights. It is meant to deter a burglar who might break the glass and reach inside to unlock the door. However, you will want to check your state building codes because double cylinder locks are a violation in some states due to the possible hindrance to escape in a hurry. During a fire, a person may become disoriented and blinded by smoke, becoming trapped behind their own door when they can not find the key to unlock it. Certainly an important issue to consider when choosing your deadbolt. Schlage’s newer handlesets don’t offer double cylinder as a choice, while many brands like Baldwin, Nostalgic Warehouse, Kwikset and Weslock offer fewer models or stock less of them. Safety Tip: If you have a Double Cylinder Dead Bolt, hang a key next to or above the door out of reach of the glass opening. Be sure everyone in the house knows where it is in case of emergency. Having a copy made with a red colored blank or red paint the top of the key, will make it more visible. Removing it when you have strangers or service people in the house is also a safe practice.