12 Grimmauld Place is the ancestral home of the Black family. It is located in London, in a Muggle neighbourhood, and the location is protected by a Fidelius Charm. As the house is invisible to the neighbourhood residents, the local Muggles have long since accepted the mistake in numbering which landed number 13 next to number 11.
In 1995, the home became the headquarters for the Order of the Phoenix, with consent of Sirius Black, with Albus Dumbledore becoming Secret-Keeper.