The development of a three-field rotation system for planting crops[AA] increased the usage of land from one half in use each year under the old two-field system to two-thirds under the new system, with a consequent increase in production. The development of the heavy plough allowed heavier soils to be farmed more efficiently, aided by the spread of the horse collar, which led to the use of draught horses in place of oxen. Horses are faster than oxen and require less pasture, factors that aided the implementation of the three-field system.

What fraction of land was under cultivation using the two-field system?