Planer blades can be sharpened or are resharpenable and in block use pressure springs to position the blade to define the cutting circle by setting gauge. Planer blades come in different materials HS, HW and coated. They can fit into alumiumin bodies and steel bodies. If you're looking at the exterior or "bark" side of the board, these arrows point uphill. On the interior or "heart" side, they point downhill. Knowing this – and which way your cutterhead spins – you can determine how to feed the board into the planer. A lot of planer blades are universal, as long as they can be held by the proper position (these are usually have only one edge, can be resharpened and the depth of cut can be adjusted).