How big is the square on a basketball backboard?

The square on a basketball backboard that helps players guide banked shots measures 18 inches vertically and 24 inches horizontally. The lines of the square must be white, 2 inches in width and centered above the rim.

As per NBA rules, the entire backboard must measure 3.5 feet vertically and 6 feet horizontally, but the NCAA does permit 4-foot-high boards. To prevent fans from having an obstructed view, the surface of the backboard must also be transparent, with the exception of the white square. The upper edge of the rectangle's bottom line is even with the rim, which sits 6 inches from the plane of the board.