Briefly state how the cash book is both journal and a ledger?

Transactions are recorded directly from source documents in the Cash Book, so there is no need to record transactions in the Journal book. Further, on the basis of the cash transactions recorded in the Cash Book, cash and bank balances can be determined, and so there is no need to prepare cash account (which is a part of ledger) separately. Thus, the Cash Book serves the purpose of both Journal as well as ledger.