Open the back of a mass-produced frame and you'll find the corners held by V-nails - small metal wedges fired into the mitre by a pneumatic gun, with a bead of fast-curing glue. It's quick, and for a few years it's fine.
Wood, though, never stops moving. Every winter the moulding shrinks across the grain; every summer it swells. The V-nail doesn't move with it, so the joint works loose a fraction each cycle until the corner opens.
What a spline does differently
A spline is a thin key of contrasting hardwood let into a slot cut across the mitre. Because its grain runs across the joint, it locks both halves mechanically - and as the moulding moves with the seasons, the interlock tightens rather than loosening.
A well-made spline corner is stronger than the wood around it. The frame will fail somewhere else first - and it won't.
Every frame we build uses hand-fitted splines and hide glue. It takes longer, and it's the reason we can guarantee the corners for life.
