There are three terms found in the Domesday Book which were used to express the amount of land held by each manor, and how well that land was utilized, so as to estimate the annual rates of taxation upon each lord.

These terms are carucates, bovates, and plough. The carucate was equivalent to approximately one hundred twenty acres, the bovate represented about fifteen acres, and the plough was reckoned to be sufficient for half a carucate of land, or sixty acres. So, if a lord had a lot of land, say two carucates, or approximately 240 acres, but only three ploughs, then at any given time one quarter of his land would lie fallow, and could not be worked. Therefore, taxing him on the assumption that all of his arable land was actually producing income would be unfair. On the other hand, taxing a lord strictly on the basis of how many ploughs he had could also be unfair, because there was always the possibility that a lord with a lot of extra land might lease some to a neighbor who had more ploughs than land available for those ploughs to work.

By recording the amount of land available and the tools with which it was worked, it became a simple matter for a tax collecter to visit once a year and estimate how well the lord had done that year and set the taxation rate accordingly.