Books of the (_a_) class are known as _principal books_ or _books of record_; that is, they contain the final or permanent record of an account. Books of the (_b_) class are of two kinds: (_ba_) books of _original entry_; (_bb_) _auxiliary books_. Any book which contains the first (or original) record of a transaction is a book of original entry (_ba_). [Illustration] [Illustration: A CORNER IN THE OFFICES OF THE PLATT IRON WORKS, DAYTON, OHIO] Any book in which records contained in books of original entry (_ba_) are assembled, to be transferred later to principal books (_a_), is an auxiliary book. Any book used for the purpose of assembling statistical information is an auxiliary book. The books most commonly used in double entry bookkeeping are: _Order Book_, _Day Book_, _Cash Book_, _Journal_, _Sales Book_, _Purchase Book_, _Ledger_. =13. Order Book.= An Order Book is a book of original entry in which is entered a record of each order or request for the shipment or delivery of merchandise. The record shows the name and address of the customer, the kinds and quantities of goods wanted, and the prices at which they are to be sold. The ruling of the order book varies according to the nature of the business. A simple form of ruling is shown. =14. Day Book.= A Day Book is a book of original entry in which are entered full particulars of each completed transaction. These records are afterwards assembled in auxiliary books, from which they are transferred to the principal books. [Illustration] The use of the day book was formerly universal, but it has been discarded by modern bookkeepers as its use involves unnecessary labor. The records formerly kept in the day book are now made directly in certain books then known as auxiliary, which makes of them books of original entry. The ruling of the day book is shown. =15. Cash Book.= A Cash Book is a book of original entry containing records of all transactions which involve either the receipt or payment of cash. The records in the cash book are in fact a complete account with cash. We debit cash for all money received, and credit cash with all money paid out; therefore, the difference between the total footings of the debit and credit sides of the cash book shows the amount of cash which we should have on hand. Since we cannot pay out more than we receive, the debit side should be the larger, unless both sides are equal, which shows that we have paid out all the cash received. The amounts entered on the debit side of the cash book are transferred (or _posted_) to the credit side of the account of the one from whom the cash is received. The amounts entered on the credit side of the cash book are posted to the debit side of the account of the one to whom the cash is paid. [Illustration]