Bifacial flaking was one of the chief methods employed by early toolmakers. It was generaly carried out by means of the technique called hard-hammer percussion. A stone serving as a hammer was held in one hand and the core, or rock to be flaked was held in the other (1) After a flake had been struck off, the core was turned so that the flake scar (here numbered to macth the flake) could serve as a platform for striking off the next flake (2). For effective flaking the angle between the striking platform and the core's outer surface must be less than 90 degrees (inset). Stroking of more flakes (3,4)yields a core with a bifaciel edge. (N.Toth, The Prehistoric Roots of a Human Concept of Symmetry, Symmetry: Culture and Science, 1, 3 (1990), 257-281)

Left: Early Acheulean cleaver and handaxe; right: Late Acheulean handaxe of the precision (and symmetry) growing by time.