In any account we can only have one half of a double entry. An account cannot be debited and credited at the same time. For example, when we sell goods for cash, cash received will be recorded on the debit side of cash book and the goods sold will be posted on the credit side of Sales Account. But in double column cash book, we have two accounts, Cash account and the bank account, so it is possible to have both a debit entry and a credit entry at the same time. For example, cash of Rs. 5,000 is deposited into a bank. In this transaction both bank account and cash account are involved and they will be recorded on both sides of Double Column cash book i.e. On the debit side in bank column and on the credit side in cash column. Thus a transaction in which Cash account and Bank account are involved, is recorded on both the sides of Double Column Cash Book, it is called "contra entry", from the Latin prefix contra meaning opposite to or against. In recording such a transaction the letter "C", is written in 'L.F' column because both aspects of the transactions are recorded and there is no need to post them into the ledger.
