there is considerable time overlap in early prehistoric stone-working industries. in some regions, acheulean tool-using groups were contemporary with other, less sophisticated industries such as the clactonian. then, later, acheulean tools occur at the same time as the more sophisticated mousterian. the acheulean was not a neatly defined period, but a tool-making technique which flourished especially well in early prehistory. it was a basic method for making stone tools which was shared across much of the old world.