Inside trim corners — cope the joint instead of mitering it

What this diagram shows

A top-down comparison of two baseboards meeting at an inside corner. On the wrong side the two pieces are each cut at 45 degrees and mitered together; because the joint depends on a true 90-degree corner, it springs open into a visible gap as the wood shrinks or because drywall mud rounds the corner. On the right side the joint is coped: the first piece runs square into the corner and butts the wall, and the second piece has its end back-cut to the moulding’s own face profile so it laps tightly over the first and the seam stays closed.