Crops were produced with the open field system. In this system, there were three large fields for the heavy and fertile land. Each field was divided into long and narrow strips. Each strip represented a day's work with the plough. One field had wheat, or perhaps rye, another had barley, oats, beans, or peas, and the third was fallow. These were rotated yearly. Each free man was allotted certain strips in each field to bear crops. His strips were far from each other, which insured some very fertile and some only fair soil, and some land near his village dwelling and some far away. These strips he cultivated, sowed with seed, and harvested for himself and his family. After the year, they reverted to common ownership for grazing. The plough used was heavy and made first of wood and later of iron. It had a mould-board which caught the soil stirred by the plough blade and threw it into a ridge. Other farm implements were: coulters, which gave free passage to the plough by cutting weeds and turf, picks, spades and shovels, reaping hooks and scythes, and sledge-hammers and anvils. With iron axes, forests were cleared to provide more arable land. The use of this open field system instead of compact enclosures worked by individuals was necessary in primitive communities which were farming only for their own subsistence. Each ox was owned by a different man as was the plough. Strips of land for agriculture were added from waste land as the community grew. There were villages which had one or two market days in each week. Cattle, sheep, pigs, poultry, calves, and rabbits were sold there. Flint workers mined with deer antler picks and ox shoulder blade shovels for flint to grind into axes, spearheads, and arrowheads. People used bone and stone tools, such as stone hammers, and then bronze and iron tools, weapons, breast plates, and horse bits, which were formed from moulds and/or forged by bronze smiths and blacksmiths. Weapons included bows and arrows, flint and copper daggers, stone axes, and shields of wood with bronze mountings. The warriors fought with chariots drawn by two horses. The horse harnesses had bronze fittings. The chariots had wood wheels, later with iron rims. When bronze came into use, there was a demand for its constituent parts: copper and tin, which were traded by rafts on waterways and the sea. Lead was mined. Wrought iroin bars were used as currency. Corpses were buried far away from any village in wood coffins, except for Kings, who were placed in stone coffins after being wrapped in linen. Possessions were buried with them.