(1) It does not mean that the price level changes with the absolute quantity of money, independently of growth of population and of the corresponding growth in the volume of exchanges. (2) It is not a mere per capita rule to be applied at a certain moment to different countries. For example, Mexico may have $9 per capita and the United States $35, while average prices may not differ in anything like that proportion. But in these two countries not only the amounts of exchanges per capita but the methods of exchange and the rapidity of the circulation of money differ greatly.[7] (3) It cannot be applied as a per capita rule to the same country through a series of years, without taking account of the many changing factors. It is estimated that in 1800 the money stock was about $5 per capita in the United States, and in 1914 about $35[8], but average prices have not necessarily changed in the same ratio. In a period of years a country may change in a multitude of ways, in complexity of industry, modes of exchange, transportation, wealth, and income. These changes require, some larger, others smaller, per capita amounts of money to maintain the same level of prices. For example, the substitution of cash payments for book-credit in retail trade calls for a larger per capita stock of money; whereas an increased use of banks and checking accounts, by economizing the use of money, enables a smaller amount of money to maintain the same level.[9] (4) Tho applied originally to standard money, the quantity theory applies to all other kinds of money circulating side by side and at a parity of value, so far as these fulfil the definition of money and are not merely supplementary aids of money. These substitutes for, or supplements to, money enable each dollar to do more work, to circulate more rapidly. If the standard money alone were doubled in quantity, while the various forms of fiduciary money (smaller coins, bank notes, government notes) remained unchanged, the quantity of money as a whole would not be doubled. Indeed, in such a case, the method of exchange would be greatly altered. According to the quantity theory, therefore, prices would not be expected to double. § 12. #Practical application of the quantity theory#. Despite the number of changing factors affecting the methods of exchange and the amount of business, the quantity theory is a rule unable at any moment. These various factors change slowly, and the quantity theory answers the question: What general change occurs in prices as a result of the increase or decrease of the money in a given community at a given moment? Like the law of gravitation and the law of projectiles, the theory must be interpreted with relation to actual conditions.