Oft times when trying to plough a dadoe in a one-by-twelve or any other kind of solid timber, you will find that the plank is bowed across its short dimension. If you don't take this out the dadoe will be very much deeper at each edge and only the correct depth in the center of the board.

To correct this I made up a simple jig of a piece of 3/4" oak with a hole drilled edgewise through the center. A long drywall screw is inserted into this hole with a couple (or more) washers under the head to protect the oak and to adjust the depth of the protuding tip of the screw. (This so that it does not fully penetrate the board to be dadoed) By placing the oak jig across the board (as shown in the photo) and screwing down the drywall screw, the bow is completely removed from the plank. If you are going to do a lot of dadoes (as for a bookcase for example) then you will need to move it along as you go.

The picture actually shows one side of a bookcase here.

I try to place the jig where there is going to be a dadoe so that the screw hole does not show, though these are easily filled if that is not possible.