All the cash transactions of a business entity are recorded for the first time in the cash book,it is , therefore, called as books of original entry or Subsidiary book. Since cash book serves the purpose of cash account as well, which is a part of ledger, so no separate cash account is prepared. Thus cash book is a part of ledger as well. As ledger is a principal book of accounts of a firm so cash book is both Subsidiary book and a principal book.