2. The furlong (meaning furrow length) was the distance a team of oxen could plow without resting. This was standardized to be exactly 40 rods.
3. An acre was the amount of land tillable by one man behind one ox in one day. Traditional acres were long and narrow due to the difficulty in turning the plow.
4. An oxgang was the amount of land tillable by one ox in a plowing season. This could vary from village to village, but was typically around 15 acres.
5. A virgate was the amount of land tillable by two oxen in a plowing season.
6. A carucate was the amount of land tillable by a team of eight oxen in a plowing season. This was equal to 8 oxgangs or 4 virgates.
In most of the Danelaw counties, the public obligations were assessed in carucates and bovates. The word carucate is derived from caruca, Latin for a plough; bovate from bos, Latin for an ox. Since the standard Domesday plough team was drawn by eight oxen, the carucate contained eight bovates. An eight-oxen plough team could plough one ploughland in the course of an agricultural year. Carucate, bovate, ploughland, and plough team were thus conceptually linked, and all derived from agricultural processes.
Domesday Book, however, makes it clear that the real world bore only a rough and ready approximation to these Platonic ideals. There were many estates where teams exceeded ploughlands, or ploughlands exceeded teams; and carucates and ploughlands were often related in an artificial manner. Many of these anomalies were due to royal grants and exemptions which had distorted the original assessments. Domesday Book itself records sweeping reductions in the assessment of estates in many counties. But as J.H. Round demonstrated more than a century ago, the system was artificial from its inception. Assessments were arrived at by allocating a round number of units to a county, then dividing them among the constituent Wapentakes, then further subdividing these among the vills in each Wapentake. Assessments arrived at in this manner could never be more than approximately equivalent to each other, or to any given area. In no sense could they be considered to be exact measurements.
For further information, see J.H. Round, Feudal England (1895); F.W. Maitland, Domesday Book and beyond (1897); Reginald Lennard, ‘The origin of the fiscal carucate’, Economic History Review, first series, vol. 14 (1944-45), pages 51-63; and Cyril Hart, The Danelaw (1992).