Well this is a unique thing. This mortising and grooving cutter set has a unique feature. It comes with several shims that allow you to (slightly) adjust the position of the tongue and groove to better suit the size (thickness) of the wood you are working with. You can adjust the position of the cutting blades by up to 1mm using all the shims provided. This may not sound like much, but it can save you a lot of sanding or planing work. The cutting edges are made of hard metal and are quite sharp.