Some advice from a cabinet maker friend - "You can't make rectangles out of trapezoids."
Even when you buy S4S (surfaced on four sides) stock, there's no guarantee that it's straight or flat. The face and back of the board may not be parallel. The edges may not be parallel. The edges may not be square to the top and/or bottom. It's up to you to check to make sure what you think you have and what you actually have are the same.
If things aren't flat, if "parallel sides" aren't parallel, if "square corners" aren't square, if "straight edges" aren't straight then things can go all to hell AFTER you've cut everything and start putting them together. Forget about mitered corners, mortise and tenons, dovetails etc. - get the stock prepared FIRST.
We're talking wood here and wood "moves". It can Cup and Bow and Crook and Twist. So - check your wood.
Set a piece on edge on a known flat surface and see if it's cupped or twisted..
Lay it face down flat on a known flat surface and alternately press down on the center of each end (1 &2). Does it rock? If it does then the board is bowed. Turn it over and try again. Now does it rock? If it does it's bowed.
Alternately press down on diagonal corners (A&B). Does it rock? Try the other corners (C&D). Now does it rock? If it rocks when you alternately press down on A & B but not when you atlernately press down on C & D the board is twisted. If it rocks when you press on both pairs of diagonal then it's cupped. If it doesn't rock then turn the board over and check again, it could be cupped and you had the concave side down
Next, set the board on edge and alternately press down on the ends. If it rocks the board is crooked and the convex side is down. If it doesn't rock rock, turn it over and check the other edge.
Now you know which boards are cupped, bowed, twisted or crooked.
YOU DON'T KNOW
- IF THE FRONT AND BACK ARE PARALLEL
- IF THE EDGES ARE PARALLEL
- IF THE EDGES ARE SQUARE TO THE FACE