...Flat and SQUARE. Once flat, joint the 'glue' edge of a stile to exact 90deg. Rail ends have to be square, or I can call myself a Cooper (things are going to curve!). And my work got a lot better/easier, when I discovered the joy of checking my miter saw for dead-nut 90deg - both blade<>fence and blade<>table - on every setup. A 4" machinist's square is a wonderful thing.