158 NATURE OF CAPITAL AND INCOME [CHar. IX of articles consists; canceling by the oblique lines, we have left but one item, $110,000, representing the « wear” of the suits. The two methods must not be confused. When we find by the method of couples that the net income of $110,000 consists exclusively of the use of suits of clothes, this does not imply that this net income is all due to the stock of clothes. To discover to what it is due, recourse must be had to the method of balances. We thereby see that only $10,000 of it is due to the stock of clothes, the remainder being due to the other capital instruments in the table and most of all ($50,000) to the logging camp. Combining the results of both methods, we may state that the total net income from the specified group of instruments consists of $110,000 worth of “ wear » of suits and that this is due partly to the stock of clothes and partly to other capital. The two methods, that of balances and that of couples, correspond in a general way to the two methods for canceling liabilities and assets in capital accounts. The method of balances gave, it will be remembered, the amount of capital belonging to each individual ; the method of couples showed of what elements the total capital con- sisted. §9 We have now followed the cancellations to which inter- actions lead, whether they be interactions of exchange or production. The case of exchange, however, needs further consideration. Since every exchange consists of two trans- fers, and every transfer of two items, a credit and a debit, the exchange evidently consists of four items in all, two of which are credits and two, debits. These four may be paired off in two ways, only one of which has thus far been mentioned. They stand, as it were, at the four corners of a square, as in the following scheme, which shows the credits and debits involved when goods worth $2