Existing within the pages of the Halliwell sister's Book of Shadows, the spell "To Create a Door" allows a witch to create a door where none exists; the witch must draw a door upon a clear space on a wall or other space with white chalk and simply knock, after which a door will open allowing the witch access through.
The spell was seen and used only once when the Halliwell sisters Prue, Piper and Phoebe traveled to 2009 to learn the reason behind Phoebe's Premonition wherein which she saw her self imprisoned and burned alive. Prue and Piper came across the spell in the Book after they retrieved it from Buckland Auction House and returned to Halliwell Manor.
The spell was amongst many pages of new spells, most of which Prue viewed as personal gain spells. Folded on top of this spell was a map of Hobart State Penitentiary, where Phoebe was being held; Prue commented it must have been a plan their future selves had in order to break Phoebe out of prison. Prue and Piper then travelled to the penitentiary where they came across an immence concrete wall; Piper drew a door on the wall, they recited the spell and knocked. A door opened and the two sisters proceeded through on a mission to save their sister.
When you find your path is blocked,
All you have to do is knock.