Frame and Panel

Frame-and-panel construction surrounds a wider center panel with a frame of narrower rails and stiles, joined at the corners (traditionally with mortise and tenon, or commercially with a cope-and-stick joint cut on a router table), with the panel captured in a groove running around the frame's inside edge. It's the standard construction behind cabinet doors, wainscoting, and traditional case-furniture sides and backs.

The entire point of the construction is managing wood movement: a solid panel that wide, built as one flat glued-up slab with no frame, would be constantly fighting seasonal expansion and contraction across its whole width. Left to float unglued in the frame's groove, the panel can expand and contract freely without stressing the frame or splitting itself, while the narrower frame members -- which move far less across their own width -- stay dimensionally stable holding everything together.