A thickness planer (also known in the UK and Australia as a thicknesser or in North America as a planer) is a woodworking machine to trim boards to a consistent thickness throughout their length. This machine transcribes the desired thickness using the downside as a reference / index. So, to produce a completely straight planed board requires that the down surface is straight before planing. Obtaining the first flat side requires either face planing with a jointer or face planing using a planer and jointer sled.