What type of lock is operated with a key from both the inside and outside?

A lock operated with a key from both the inside and outside is characterized by its dual-key mechanism, which enhances security by requiring a key for both access points. The double cylinder deadbolt fits this description perfectly. This type of deadbolt is particularly effective in preventing unauthorized entry, especially in situations where a glass panel is nearby. With a double cylinder deadbolt, even if a burglar breaks a window next to the door, they would still need a key to unlock the door from the inside.

In contrast, a single cylinder deadbolt only requires a key on the outside while allowing for a thumb turn on the inside, making it less secure in the presence of nearby glass. A mortise lock may also incorporate a key mechanism but typically has a more complex installation and additional features that differentiate it from a straightforward deadbolt, usually suited for commercial applications. A padlock, while operable with a key, does not fit the description of being attached to a door in the same manner as the other options, as it is typically a portable locking device used for various applications. Thus, the double cylinder deadbolt is the correct answer for a lock that is operable with a key from both the inside and outside.