A double-cylinder deadbolt is a type of lock that requires a key to operate from both the inside and outside. This design enhances security by preventing unauthorized entry through a window or glass door near the lock, as intruders cannot simply turn the knob. However, double-cylinder deadbolts can pose safety risks in emergencies, as they require a key to exit. This can be problematic if a quick escape is needed during a fire or other urgent situation. Therefore, it's essential to weigh the security benefits against potential safety concerns when considering this lock type for your home.