Ikea's Billy shelf uses the cardboard back insert to prevent fore and aft movement of the shelf. My wife wanted an open back shelf. These brackets secure the shelf to the backing slot and to the shelves with a #6 or #7 wood screw (this was from a few years ago, so let me know if that is incorrect).

• no supports, print in orientation shown
• 40% + infill
• 0.20 mm layer height