_The fundamental principle of double entry is that there must be a corresponding credit for every debit._ _Example_--When we sell John Doe two tons of coal, we debit his account; but we have decreased the value of our stock of coal, and to complete the double entry, we credit coal account (or _Merchandise_, as the account representing our stock in trade is sometimes known). When he pays us money, we credit his account, and debit cash. =10. Advantages of Double Entry.= The principal advantages of double entry bookkeeping are that the system permits of making an accurate exhibit of the standing of the business; it exhibits the profits and losses; it shows the sources of profits and the causes of losses; it permits of proof of the accuracy of the records. [Illustration] 11. _Account books_ are ruled with special forms which adapt them to bookkeeping records. The forms of ruling are many and varied to suit the requirements of different classes of business. Rulings for double entry bookkeeping do not differ materially from those used in single entry. For double entry, at least two amount columns must be provided--one for debits, and one for credits. The most common form of ruling is known as _journal ruling_. A book with this ruling is also known as a _journal ledger_. The words in parentheses explain the purpose of the different columns. The abbreviations _Dr._ (debit) and _Cr._ (credit) are sometimes written at the head of the amount column; but most bookkeepers omit them, as the position of the columns indicates their purpose. DEMONSTRATION To illustrate the manner of entering transactions in accounts, we show in the accompanying diagrams how the transactions used in the foregoing examples would appear in the proper accounts. [Illustration] EXAMPLES FOR PRACTICE 1. On journal ruled paper, which can be procured at any stationer's, write up the account of John Doe as per example given in Articles 2 and 3. 2. Write up the account of John Doe, showing also the accounts necessary to complete the double entry, as per example in Article 11. 3. Write up the accounts covering the following transactions, by the single entry method: Nov. 16. Sold to James Stevenson 4 cords of wood, at $4.75 per cord. Sold to Andrew White 2½ tons of coal, at $6.00 a ton. Nov. 17. Sold to Wm. Johnson 1 ton of coal, at $7.00 a ton. Nov. 19. Received from James Stevenson $12.00 cash, to apply on account. Nov. 20. Received from Wm. Johnson $7.00 in payment of account. 4. Write up the same accounts by the double entry method, using a merchandise account to represent all classes of merchandise sold. CLASSES OF ACCOUNT BOOKS 12. Account books are of two classes: (_a_) those in which complete records of transactions, or complete accounts, are kept; (_b_) those which contain particulars of individual transactions which must afterward be transferred to books of the (_a_) class.