In the 1930s, when Keyhouse Manor was invaded by gangsters, Jean Locke had Louis Dassin follow her through the Gender Changing Door. This switched their genders and scarred Louis who ran away from the Lockes permanently female.
The key allows a person to become the opposite of their current gender when this key is used with the Gender Changing Door.
The Gender Door is a half-sized door that connects Tyler and Kinsey's bedrooms.
"My sister - or should I now say my brother!-fights the shadow war with Crais in the streets of Boston whilst I wait at home, like a helpless maiden, praying to the ALLMAIGHTY! for her safe return. When first I fashin'd the key, I imagained she maight trainsform to a boy to protect her, if necessaire, from the unsavorie lusts of ENGLISHMEN should the King's foot-soldiers return to Lovecraft to abuse God fairing womain. Never did I think she wouldst WILLENGLY caist off the wardrobe of her femininitie for this ruggaid liberation among men..."