Explanation

The cash book is unique because it serves as both a subsidiary book (book of original entry) and a ledger. It records all cash and bank transactions and also contains the cash and bank accounts themselves.

As a book of original entry, the cash book is where cash and bank transactions are first recorded when they occur. As a ledger, it contains the cash account and bank account balances, eliminating the need to maintain separate accounts in the general ledger.

The general journal is only a book of original entry for transactions that do not fit elsewhere. The purchases day book records credit purchases only. The returns outward book records goods returned to suppliers. None of these also function as ledgers.

This dual nature makes the cash book efficient because cash transactions are recorded directly in the ledger account, saving the step of posting from a journal to a ledger.