Double entry is the name given to the method of recording the transactions so that the dual aspect concept is upheld.
The Daybook transactions are “posted” to the ledgers. So for example the total sales for the day from the Sales Daybook is written up in the Sales Account of the Nominal Ledger and the individual amounts are written up on the individual customers’ account on the Sales Ledger. In doing so we are creating the double entry. That is we have an entry in the Sales Account (Nominal Ledger) and an equal (and opposite) amount in the customers’ accounts in the Sales Ledger.