The three field- system replaced the two-field system in Europe during the Middle Ages.
In the traditional two-field system one field was used for the sowing of crop, while another field of equal size was left fallow. The use of the two fields was rotated during the following year.
In the three-field system the sequence of field use involved an autumn planting of grain (wheat, barley or rye) and a spring planting of peas, beans, oats or barley. This reduced the amount of fallow fields to one third. The legumes planted in spring improved the soil through the fixation of nitrogen. At the same time the variation of crops improved the peasants' diet.
The three-field system was possible wherever there were reliable summer rains. Having two harvests per year instead of one gave better protection against crop failure and famine. The additional harvest of oats allowed the replacement of oxen as work animals by the more agile horses. This led to the invention of the padded horse collar that allowed horses to be used for pulling heavy cartloads.