What is a single cylinder deadbolt?

• A. Key on both sides (interior and exterior)
• B. Thumb turn on interior
• C. Key on exterior and interior
• D. Key only on the exterior

Answer: (D)

A single-cylinder deadbolt is designed so you can unlock it from the outside with a key and operate it from the inside without a key, usually with a thumb turn. This configuration provides security from the outside while letting you exit quickly from inside.

The description that matches this design is the one that states there’s a key on the exterior and some means of interior operation (like a thumb turn). It’s the standard setup because it combines external key access with simple, keyless interior control for everyday use and safety.

Descriptions that imply both sides require a key, or that interior operation isn’t possible, don’t fit the typical single-cylinder design. Likewise, a description that mentions a key only on the exterior would ignore the interior accessibility that makes a single-cylinder deadbolt practical for most doors.