It may sound obvious, but if you want your cabinet doors to swing freely and stay true, they have to be flat. And if you follow the proper techniques, they will be. For a basic frame-and-panel door, the process starts with choosing straight stock and milling it carefully. Then, you have to use the right orientation when cutting both mortises and tenons, to ensure that the doors will be flush. A bit of fine-tuning and the proper glue-up technique, and you are on your way to flat doors, every time.