In this way, when books on the double entry system have all the sums on the debtor side and all the sums on the creditor side added up, the total amounts in both cases are the same. That is, if the books have been rightly kept and no mistake has been made in addition, like that of the man who spent a long time trying to make them come right, and found at last he had made the slight mistake on one of the sides of adding in the figures of the current year. The object of double entry is to establish a series of checks so that mistakes are not likely to occur, and in all establishments of any importance this is the system adopted. Books kept by the other and simpler system of single entry afford no check upon themselves. “Errors in addition,” says Mr. A. L. Lewis, “which are as easy to make in hundreds of pounds as in pence, errors and omissions in posting or in carrying forward balances, any or all of which may entail serious loss, can only be prevented in single entry books by the most careful checking and rechecking every item, and no one, however sharpsighted, can always avoid making an error, and even failing to discover it when made.” What is called _posting_ in book-keeping is the operation of transferring items from one book to another, and arranging them there under their proper heads. The difference between the Dr. and Cr. sides of an account is known as the _balance_. Transactions are entered in their books by business people at once. They never put off making an entry till to-morrow, for they are well aware that there is no putting any dependence on memory. They are constantly turning over their books, too, so as to keep their affairs fresh in their minds, and see in a general way how they are getting on. Then every little while they go particularly into all their accounts and strike a balance as it is called—that is to say, make out a statement of their assets and liabilities, and arrange things for a fresh start. The word assets, we may as well mention, stands for property or sums of money owing to anyone, and liabilities means just the reverse.