Room of Requirement

"Sometimes it is there, and sometimes it is not..."
-- Dobby
The Room of Requirement is a magical room which can only be discovered by someone who is in need. It is called the Come and Go Room by the house-elves of Hogwarts. The Room is located on the seventh floor, opposite a tapestry showing Barnabas the Barmy trying to teach trolls to dance the ballet. To make the Room appear, a person has to walk past the section of blank wall three times concentrating hard on what is needed.