The mediaeval English acre was a long narrow strip forty rods in length and four rods in width, a half-acre or quarter-acre being of the same length, but of two rods or one rod in width. The rod was of different lengths in different parts of the country, depending on local custom, but the most common length was that prescribed by statute, that is to say, sixteen and a half feet. The length of the acre, forty rods, has given rise to one of the familiar units of length, the furlong, that is, a "furrow-long," or the length of a furrow. A rood is a piece of land one rod wide and forty rods long, that is, the fourth of an acre. A series of such strips were ploughed up successively, being separated from each other either by leaving the width of a furrow or two unploughed, or by marking the division with stones, or perhaps by simply throwing the first furrow of the next strip in the opposite direction when it was ploughed. When an unploughed border was left covered with grass or stones, it was called a "balk." A number of such acres or fractions of acres with their slight dividing ridges thus lay alongside of one another in a group, the number being defined by the configuration of the ground, by a traditional division among a given number of tenants, or by some other cause. Other groups of strips lay at right angles or inclined to these, so that the whole arable land of the village when ploughed or under cultivation had, like many French, German, or Swiss landscapes at the present time, something of the appearance of a great irregular checker-board or patchwork quilt, each large square being divided in one direction by parallel lines. Usually the cultivated open fields belonging to a village were divided into three or more large tracts or fields and these were cultivated according to some established rotation of crops. The most common of these was the three-field system, by which in any one year all the strips in one tract or field would be planted with wheat, rye, or some other crop which is planted in the fall and harvested the next summer; a second great field would be planted with oats, barley, peas, or some such crop as is planted in the spring and harvested in the fall; the third field would be fallow, recuperating its fertility. The next year all the acres in the field which had lain fallow the year before might be planted with a fall crop, the wheat field of the previous year being planted with a spring crop, and the oats field in its turn now lying uncultivated for a year. The third year a further exchange would be made by which a fall crop would succeed the fallow of that year and the spring crop of the previous year, a spring crop would succeed the last year's fall crop and the field from which the spring crop was taken now in its turn would enjoy a fallow year. In the fourth year the rotation would begin over again. [Illustration: Village with Open Fields, Udenhausen, near Coblentz, Germany. (From a photograph taken in 1894.)]