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. These deadbolts are commonly used in residential and commercial properties where additional security is needed. However, they can pose a safety risk in emergencies, as occupants may need a key to exit quickly. Therefore, it's essential to consider the balance between security and safety when choosing a locking mechanism for your property.