Cut inside corners by coping for a tight, flush fit and cut outside corners with precise miter joints for a clean, professional look.

For inside corners, start by installing the first board square and cut the mating board with a coping saw so the profile fits snugly against the face of the first board; hold the coped piece against the installed board and check for gaps, then trim small high spots with a file for a perfect fit. When coping, cut slightly proud and remove material from the back of the profile only so the face profile fully contacts the installed board; keep the coping saw blade perpendicular to the face and follow the profile curve slowly to avoid overcutting. For outside corners, measure twice and cut both boards at forty five degrees with a sharp saw set to the same angle; dry fit before nailing and make tiny back cuts to remove paint or finish that prevents a tight joint. If walls or floors are out of square, cut the miter slightly long and use a shooting board or block plane to tune the angle until the joint closes evenly; small gaps can be reduced with wood glue squeezed into the joint and clamped or pinned, not just nailed. Use caulk sparingly along the top edge only to hide minor gaps and prevent cracking later, and avoid caulking the joint between two pieces of baseboard where movement can split the seal; match caulk color to paint or use paintable silicone. Always fasten into studs or use finishing nails into the top and bottom of the profile, countersink nails slightly and fill with matching wood filler, then sand and paint to blend the joint seamlessly.

Common mistakes are relying on mitering both sides in an inside corner which leaves visible gaps when walls are not perfectly square, so cope the second piece instead and tune the fit with a file for a flush joint. Another issue is cutting outside miters without checking wall angles, which causes long gaps at the top or bottom, so measure the actual corner angle with a bevel gauge and transfer that angle to your saw or adjust the miter by shaving until the joint closes. A third problem is overcaulking or using caulk to hide large gaps, which looks messy and fails with movement; instead glue and clamp or pin the joint, fill tiny remaining voids with paintable caulk, and paint promptly so the repair blends.

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