What causes gaps in 45-degree miter joints on a keepsake box?

[Victoria.A]

I’ve been trying to build a small wooden keepsake box with mitered corners, but I can’t get the joints to close up perfectly without a tiny gap on one side. My miter saw is set to 45 degrees, and I’m using a clamp, but something’s still off.

[MiaM]

I had a tiny gap like that once. I found my saw wasn't really 45 after I dropped the fence and the blade looked fine. I re-zeroed the saw, tested with a short piece, and confirmed both sides cut true. After that, the joint closed up.

[SofiaBT]

Stock can bite you here too. If the ends aren’t square or the board is cupped, the joint never seats. I ran a quick test: two scraps, checked with a square, and trimmed until I could seat both corners flush.

[Daniel.M]

I tried glue and clamps and still had a thin gap on one side. I ended up using a thin wedge behind the joint while clamping to take up that tiny bit of loss. A spline along the seam helped on a later box, too.

[Ella.M]

Have you checked if you're cutting both miters in the same direction (left vs right) or if one piece is flipped before gluing? Could be that one miter is mirrored.