Mills ‘Saddle-querns’, in which grain (see cereals) was rubbed between a fixed flat lower stone and a smaller hand-held upper stone, had been in general use for thousands of years before the ‘hopper-rubber’ mill appeared in Greece by the 5th cent. bce. Mechanized versions consisted of a rectangular upper stone, with a cavity that acted as a hopper for grain, pivoted at one end to allow a side-to-side action; grooves cut into the grinding surfaces improved the flow of grain and the removal of flour from the lower stone. Perhaps as early as the 3rd cent. bce, the introduction of a pair of round stones made a dramatic improvement, for a central (adjustable) pivot took the weight of the upper stone, which could be moved in a continuous rotary motion, assisted by its own momentum, and propelled by a crank-like vertical handle set into the upper surface. This development did not take place in Greece, for rotary mills did not appear there before the Roman period. Rotary mills were also scaled up into the hourglass-shaped ‘Pompeian’ form, powered by animals or slaves, in contexts such as commercial bakeries.