The working of a primary flake, usually by the removal of small fragments, to form a tool; to thin, sharpen, straighten, or otherwise refine an existing stone tool for further use. It is the work done to a flint implement after its preliminary roughing-out in order to make it into a functional tool. In the case of a core-tool, such as a hand-ax, retouch may consist of roughly trimming the edge by striking with a hammerstone, but on smaller, finer flake or blade tools it is usually carried out by pressure-flaking. It is done two ways, either by blows that knock small flakes off an edge (percussion retouch) or by pressure to force the flakes off (pressure retouch). The different types of retouch are also described as: backing or blunting retouch, and invasive or normal retouch. Invasive retouch can be steep or shallow, depending mainly on the kind of edge being retouched; this retouch can also be scaly in character. Backing is most often applied to blades and may have been done to blunt the back or to bring its end to a stout point. Evidence suggests that it may have been done to regularize the blade edge to facilitate fixing by resin 'mastic' to a bone or wood shaft. Such a strip of mastic was found in Lascaux, France. Notching or toothing is another form of retouch, and the removal of spalls or slivers as in the burin technique could be regarded as a further form of retouch or modification. Retouch is one of the most obvious features distinguishing a manmade from a naturally struck flint.