The Wealth of Nations by Part 5 Chapter 2 Page 193

In the fourteenth of Richard II this duty was raised to one shilling in the pound, but three years afterwards it was again reduced to sixpence. It was raised to eightpence in the second year of Henry IV, and in the fourth year of the same prince to one shilling. From this time to the ninth year of William III this duty continued at one shilling in the pound. The duties of tonnage and poundage were generally granted to the king by one and the same Act of Parliament, and were called the Subsidy of Tonnage and Poundage. The Subsidy of Poundage having continued for so long a time at one shining in the pound, or at five per cent, a subsidy came, in the language of the customs, to denote a general duty of this kind of five per cent.

This subsidy, which is now called the Old Subsidy, still continues to be levied according to the