The Sixth Key was held by Superior Saturday for over 10,000 years, until Lord Arthur claimed it. It is shown that only the Sixth Key was stopping the Piper from taking control over Superior Saturday herself. The Sixth Key grants control over the Upper House. The Sixth Key takes the form of an unassuming quill pen.
The Powers of the Sixth Key are numerous, but mostly unknown. The known Powers of the Sixth Key are:
- Opening and Closing
- Locking and Unlocking
- Holding Back/Banishing Nothing
- Manipulating Objects (by writing on them)
- Accessing the Improbable Stair
- Using spells on a supreme level; The spells Superior Saturday uses in conjunction with the Sixth Key are capable of killing hundreds of the Guardians of the Incomparable Gardens.
As of Lady Friday it is said that all Keys can kill anything the wielder wishes, be it Nithling, Denizen, Superior Denizen or Mortal. It is beleived that any being not of those categories, such as The Architect, The Old One and their three children (Lord Sunday , The Mariner, and The Piper) would be unphased by any such attempts. Towards the end of the series, it is thought Lord Arthur would be immune as well. It is unknown if the Trustees were immune while wielding their Keys.