Kitchen cabinets come from the manufacturer in standard sizes. Installers position the cabinet sections to the kitchen's walls to fit the individual kitchen. Kitchen cabinets use a filler piece to cover the gap between the edge of the cabinet and the wall, or the space between two narrow cabinets. Cabinet installers custom cut each filler piece from a flat board with the same finish as the cabinets. The filler pieces give the cabinets a solid-looking face across its entire length.
Install the cabinets in the kitchen in their final location. Do not set the countertops on the cabinets.
Measure the top and bottom of the gap between the cabinets or between the cabinet and the wall with a tape measure. Transfer these measurements to a piece of cabinet filler, placing pencil marks at the appropriate spots. Draw a straight line between the two pencil marks, using a framing square as a straightedge.
Cover the finished side of the filler piece next to the pencil line with masking tape. The masking tape protects the filler's surface while cutting off the excess.
Cut the filler piece along the pencil line with a jigsaw equipped with a fine-tooth wood blade. Keep the saw's blade guard on the masking tape. Test-fit the filler piece and trim as needed.
Pull off the masking tape and remove the sharp edges from the filler's cut edge with sandpaper.
Slip the filler piece into place. Grip the cabinets on both sides of the filler piece with two C-clamps. Place one C-clamp near the top of the filler piece and one near the bottom. Tighten the C-clamps until the cabinets are tight against the filler piece. The top edge of the filler piece should match the tops of the cabinets and the filler piece's face should line up flush with the cabinets' face.
Drill three pilot holes through the inside of the cabinet into the filler piece's side with an electric drill equipped with a 1/8-inch drill bit. Place one hole 3 inches from the top, another 3 inches from the bottom and the last in the middle.
Secure the filler piece to the cabinets with 1½-inch wood screws. Run a screw through each pilot hole with the electric drill. Remove the C-clamps.