You first want to flatten one side on a jointer and then plane the other side on a thickness planer. A jointer can take the bow or cup out of a board and make one surface flat but it cannot make the opposite face parallel to the first face. That's where the thickness planer comes in. If you run this board only through a thickness planer you will just end up with a thinner board that still has a bow in it. I hope this answers your question.
Ken is dead on. You can plane the board by hand or use a jointer. There was a time when planed board using a router. Whatever it takes to get one side flat. then the planer will flatten the other side.
I use a piece of plywood as a sub sled and place the bow down so the edges are firmly on the plywood and then run it through the planer till flat and then flip it over and run them through again. Only reason I do this is I do not have much practice with a jointer yet.
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