Silver Key

The Silver Key is an object found in the dresser drawer of San's room. It is used to unlock San's workshop, located in the back side of his house.

The Silver Key can be acquired only in the Neutral Route if the protagonist gained no EXP, as entering Sans's room is needed to obtain it. To get the key to his room, the protagonist must recieve San's judgement in the Last Corridor, reload the game without saving, talk to him again and repeat the previous steps three times.

Trivia

 * The fact that this key is explicitly silver may allude to H.P. Lovecraft's short story The Silver Key, which focuses on time travel.