The size of the strips of land in the arable fields varied, but was generally an acre, in most places a furlong (furrow long) or 220 yards in length, and 22 yards broad; or in other words, 40 rods of 5-1/2 yards in length and 4 in breadth. There was, however, little uniformity in measurement before the Norman Conquest, the rod by which the furlongs and acres were measured varying in length from 12 to 24 feet, so that one acre might be four times as large as another.[7] The acre was, roughly speaking, the amount that a team could plough in a day, and seems to have been from early times the unit of measuring the area of land.[8] Of necessity the real acre and the ideal acre were also different, for the reason that the former had to contend with the inequalities of the earth's surface and varied much when no scientific measurement was possible. As late as 1820 the acre was of many different sizes in England. In Bedfordshire it was 2 roods, in Dorset 134 perches instead of 160, in Lincolnshire 5 roods, in Staffordshire 2-1/4 acres. To-day the Cheshire acre is 10,240 square yards. As, however, an acre was and is a day's ploughing for a team, we may assume that the most usual acre was the same area then as now. There were also half-acre strips, but whatever the size the strips were divided one from another by narrow grass paths generally called 'balks', and at the end of a group of these strips was the 'headland' where the plough turned, the name being common to-day. Many of these common fields remained until well on in the nineteenth century; in 1815 half the county of Huntingdon was in this condition, and a few still exist.[9] Cultivating the same field year after year naturally exhausted the soil, so that the two-field system came in, under which one was cultivated and the other left fallow; and this was followed by the three-field system, by which two were cropped in any one year and one lay fallow, the last-named becoming general as it yielded better results, though the former continued, especially in the North. Under the three-field plan the husbandman early in the autumn would plough the field that had been lying fallow during summer, and sow wheat or rye; in the spring he broke up the stubble of the field on which the last wheat crop had been grown and sowed barley or oats; in June he ploughed up the stubble of the last spring crop and fallowed the field.[10] As soon as the crops began to grow in the arable fields and the grass in the meadows to spring, they were carefully fenced to prevent trespass of man and beast; and, as soon as the crops came off, the fields became common for all the village to turn their stock upon, the arable fields being usually common from Lammas (August 1) to Candlemas (February 2) and the meadows from July 6, old Midsummer Day, to Candlemas[11]; but as in this climate the season both of hay and corn harvest varies considerably, these dates cannot have been fixed.