What is “three-field crop rotation”?

The story behind the answer

Medieval farmers divided their arable land into three parts: one sown with autumn grains (like wheat or rye), one with spring crops (oats, barley, or legumes), and one left fallow to recover. Each year, the uses rotated, maintaining soil fertility and boosting yields compared to the older two-field system.

This innovation, widespread in Europe from the High Middle Ages, worked hand-in-hand with the heavy plow and the horse collar. Legumes helped replenish nitrogen, oats fed horses that could plow more land, and staggering harvests reduced the risk of total crop failure. The result was more dependable food supplies, population growth, and the surplus needed to support towns, trade, and the construction of those famous medieval castles and cathedrals.