A definite sum of money given to the petty cashier in the beginning of a period and to reimburse him for payments made at the end of the period is called imprest amount in petty cash book.

If customers frequently return the goods sold to them that is called return inwards. It is recorded in return inward book.
If we frequently return the goods purchase by us from supplier that is called return outwards. It is recorded in return outward book.

As a business expends and the number of transactions because large, it become difficult to record all transaction is the journal, for quick, efficient and accurate recording of business transactions, journals each meant for recording all transction of similar nature e.g. Cash Book, Purchases Book etc.

Following are the important special purpose subsidiary books :
(i) Cash Book: Only cash transactions are recorded in this book. In double column cash book, we record cash transactions as well as transactions related to the bank. In such a case Cash account and Bank account is not prepared separately because both the columns are involved in the cash book. A separate Cash Book is maintained for payment of small/petty expenses.
(ii) Purchases Book : Only credit purchases of goods and materials (required for the purpose of production) are recorded in the purchases book.
(iii) Sales Book: Only credit sales of goods are recorded in this book.
(iv) Purchase Returns Book : Goods returned to suppliers are recorded in this book. It is also called 'Returns outward book.'
(v) Sale Returns Book: Goods returned by customers are recorded in this book. It is also called 'Returns inward book'.
(vi) Bills Receivable Book : Bills drawn and accepted by debtors and promissory notes received from them are recorded in this book.
(vii) Bills Payable Book: Bills accepted by the firm and promissory notes issued to creditors are recorded in this book.
(viii) Journal Proper: The business transaction, which cannot be recorded in any of the seven books, mentioned above are recorded in this book. It is also called 'Miscellaneous Journal'.

Some subsidiary books are :
(i) Cash Book
(ii) Purchases Journal or Purchases Book
(iii) Sales Journal or Sales Book
(iv) Purchases Returns Journal or Purchases Returns Book
(v) Sales Returns Journal or Sales Returns Book
(vi) Purchases Book
(vii) Bills Receivable Book
(viii) Bills Payable Book.

Following are the important advantages of maintaining a Petty Cash Book:
(i) It saves the time of main cashier.
(ii) It helps in controlling the petty expenses.
(iii) Posting from Petty Cash Book is made at the end of a specified period. It saves the time and simplifies the posting.
(iv) When expenses paid are reimbursed to petty cashier, these are checked by the main cashier. It reduces the chances of errors and frauds.

Cash Book is both a subsidiary book as well as a principal book. As a subsidiary book, it records all the cash transactions. Ledger accounts are prepared on the basis of each book records. Thus, it is a book of original entry or a subsidiary book. Cash Book represents cash account. Cash balance is directly entered in the trial balance from Cash Book. Therefore, it is a part of ledger also. Hence, it is also treated as a principal book of accounts. The Cash Book is thus, both a subsidiary book and a principal book.