The Wealth of Nations by Part 1 Chapter 5 Page 49

As it rarely happens that these are exactly agreeable to their standard, the merchant adjusts the price of his goods, as well as he can, not to what those weights and measures ought to be, but to what, upon an average, he finds by experience they actually are. In consequence of a like disorder in the coin, the price of goods comes, in the same manner, to be adjusted, not to the quantity of pure gold or silver which the corn ought to contain, but to that which, upon an average, it is found by experience, it actually does contain.

By the money-price of goods, it is to be observed, I understand always the quantity of pure gold or silver for which they are sold, without any regard to the denomination of the coin.

Six shillings and eightpence, for example, in the time of Edward I, I consider