"Three up...two across..." he muttered. "Right, stand back, Harry."

The brick he had touched quivered - it wriggled - in the middle, a small hole appeared - it grew wider and wider - a second later they were facing an archway large enough even for Hagrid, an archway onto a cobbled street that twisted and turned out of sight.

"Welcome," said Hagrid, "to Diagon Alley." (p. 71)

Rowling, J.K.
Harry Potter and the Sorcerer's Stone,
New York: Scholastic Inc., 1997.