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.