Magic Apron
The Magic Apron is the lace apron Mrs. Yoop always wears that alone can open the doors and windows of her castle.
First named in The Tin Woodman of Oz, ch. 7
"You cannot open any door or window in this enchanted castle unless you are wearing the Magic Apron," said the Canary.
What the books say it is
made of lace 1 chapter
The lace one, which the Giantess always wears.
The Tin Woodman of Oz, ch. 7