Marking knives have a steel blade which is sharpened to a knife edge, but only on one side of the blade - the other side of the blade is kept straight so as to follow a ruler or straight-edge. The purpose of the knife edge is to sever fibres as the marking knife is drawn over them. This produces a very accurate line which aids in making accurate cuts with the saw or chisel. The blade is of tool steel with a hardwood or plastic handle.
These instruments are generally used when laying out across the grain. They are avoided when laying out with the grain as the blade tends to follow the fibres, resulting in inaccurate lines. The scratch awl is preferred for long grain layout.