A cash book o both a journal as well as a ledger. The cash book is a journal because it records the cash transactions from the source document for the first time and then these are posted in the respective ledger accounts. The cash book is a ledger in the sense that it serves the purpose of a cash account also. Wen the cash book is prepared, the Cash A/c is not required to be prepared. Thus, the cash book serves the purpose of a subsidiary book as well as the principal book.