Step 7. Install Stretchers
Use the 13 Amp Circular Saw to cut two pieces of 1 x 4 pine at 28 1/2". Apply glue to one side face and both ends of each piece.
Fit one piece under the top board of the box, flat against the back panel, and one piece resting on top of the box's bottom board, against the back panel.
Drill pilot holes and fasten the stretchers with 2" screws through both sides of the box. Use 3/4" screws randomly through the back.
Tip: Stretchers add stability to a fully-loaded bookcase and provide structure for anchoring the unit to the wall if desired.
Depending on what type of screw you're using, pilot holes should be just slightly smaller than the screw.
The idea is to allow the hole to be big enough for the screw to pass through without splitting the wood, but at the same time, small enough so that the threads grip firmly.