Mason Gaffney, in describing (accurately) the original (10% of the produce of the land) and the current (10% of income), states that 'the concept has been turned upside down'. Others might say that it has been modernized: that in the agricultural societies of old (including of 620 AD in Arabia), the produce of the land was a surrogate (and easily measurable) indicator of income, and that now, in non-agricultural societies, we use income because produce of the land is meaningless (or zero) for the many non-agricultural types. Peter G. Stillman