When the two accounts involved in a transaction are cash account and bank account, then both the aspects are entered in cash book itself. As both the debit and credit aspects of a transaction are recorded in the cash book, such entries are called contra entries.
Example:
(i) When cash is paid into bank, it is recorded in the bank column on the debit side and in the cash column on the credit side of the cash book.
(ii) When cash is drawn from bank for office use, it is entered in cash column on the debit side and in the bank column on the credit side of the cash book.

(i) Cash discount is an allowance for prompt and is based on Net Price List
(ii) An allowance given to customers is known as Discount Allowed and
(iii) An allowance made by creditors off the amount owing is called Discount Received.
Accounting treatment of discounts:
Company ABC's book have the following transactions relating to discounts:
2.03.2018 Received from Mr. ABC,      Rs. 2,900
discount allowed from XYZ is               Rs. 100
Accounting entry for discount allowed:
Debit: Bank A/c Dr.                                Rs. 2,900
Credit: Accounts Receivable Mr. ABC Rs. 3,000
Being discount allowed of 100 to customer Mr. ABC for prompt payment.

Following are the advantages of maintaining petty cash book:
(i) There can be better control over petty payments.
(ii) There is saving of time of the main cashier.
(iii) Cash book is not loaded with many petty payments.
(iv) Posting of entries from main cash book and petty cash book is comparatively easy.

Following are the difference between cash discount and trade discount.

(i) The amount given to the petty cashier in advance is known as "Imprest Money". The word imprest means payment in advance.
(ii) The word imprest means payment in advance.
(iii) The petty cashier makes payments from this amount and records them in petty cash book. At the end of a particular period the petty cashier submits the petty cash book to the head cashier.
(iv) The head cashier scrutinises the petty payments and issues a fresh cheque equal to the amount spent by petty cashier so that the total amount with the petty cashier is how equal to the amount he had received in the beginning as advance.
(v) Under the system the total cash with the petty cashier never exceeds the imprest and at any time during the period. This method thus provides an effective control over petty payments.