Harry uses Platform 9¾ at London’s King’s Cross station in J.K. Rowling’s Harry Potter books.
The hidden platform is the departure point for the Hogwarts Express. Harry first reaches it before beginning his first year at Hogwarts, when he follows the Weasley family through the barrier between platforms 9 and 10.
The fraction is written as nine and three-quarters, usually displayed as 9¾. It is a fictional platform rather than a normal railway platform, and the magical barrier is part of the wizarding world’s concealment from Muggles.
King’s Cross station is a real London railway terminus, and the books’ platform number inspired a permanent Platform 9¾ shop and photo location there. In the films, scenes involving the entrance were filmed at or near King’s Cross, though the exact visual arrangements differ from the novel’s description.