Originally, the key was narrower than it is today and had the shape of a skeleton/basic lever lock keyhole, measuring six feet (1.8 m) wide, hence "the key", with the free throw circle as the head, and the shaded lane as the body.
Men's professional basketball in the United States (notably the National Basketball Association) widened it further to 16 feet (4.9 m) in the 1964-65 NBA season to lessen the effectiveness of centers, especially Wilt Chamberlain [REF].