What is the difference between a single cylinder and a double cylinder handleset?

A single cylinder handleset has a functioning deadbolt which requires a key on the exterior side, while it can be locked or unlocked by a thumbturn on the interior side. This type of handleset is commonly used on front doors where entrance from outside and inside is necessary. On the other hand, a double cylinder handleset operates with a key on both the exterior and interior sides to lock or unlock the deadbolt. This offers added security but may also pose a safety risk during emergency situations as a key is required to exit from inside. Therefore, double cylinder handlesets are typically recommended for doors with glass panels or close proximity to windows, where the additional security is needed to prevent unauthorized access.