In calculation of the due date three extra days are added, to the specified period of the bill is called "Days of grace".

In purchase return, the goods are returned for the following reasons:
(i) Goods may be defective
(ii) They might have been damaged in transit
(iii) Quantities delivered may not agree with invoice
(iv) They might not as per the sample or specifications.
(v) There may be breach of agreement.

When a bill is drawn payable after a specified period on the date on which the payment should be made is called 'Due Date'.

(i) The seller (creditor) prepares the bill in the form presented above.
(ii) The act of preparing the bill in its complete form with the signature is known as 'drawing' a bill.

(i) At the end of the accounting year, all nominal accounts are closed but the business has to be carried on with previous year's assets and liabilities.
(ii) Journal entry made in the beginning of the current year with the balances of assets and liabilities of the previous year is opening journal entry.

Purposes of subsidiary books:
(i) Purchases Book: It records only credit purchases of goods by the trader.
(ii) Sales Book: It records only credit sale of goods by the trader.
(iii) Purchases Returns Book: It records the goods returned by the trader to the suppliers.
(iv) Sales Return Book: It records the goods returned by the customers.
(v) Bills Receivable Book: It records the receipt of bills from customers.
(vi) Bills Payable Book: It records the bills accepted by the trader.
(vii) Cash Book: It records all the cash transactions of the business. (i.e.,) the receipts and payments of cash.
(viii) Journal Proper: It is the journal which records entries which cannot be entered in any of the subsidiary books.

Rectifying entries are passed for rectifying the errors which might have committed in the books of accounts.
Example: Purchase of furniture for Rs. 10,000 was wrongly debited in Purchases A/c. To rectify the error the following entry is to be passed. 

The advantages of maintaining subsidiary books can be summarised as under :
(i) Proper and systematic record of the business transactions : All the business transactions are classified and grouped conveniently as cash and non cash transactions, which are further classified as credit purchases, credit sales and returns etc.
(ii) Convenient posting: All the transactions of a particular nature are recorded at one place, i.e., in one of the subsidiary books. For Example: All credit purchases of goods are recorded in the purchases book and all credit sales of goods are recorded in the sales book.
(iii) Division of work: As journals are sub-divided, the work will be sub-divided and different persons can work on different books at the same time and the work can be speedily completed.
(iv) Efficiency: The sub-division of work gives the advantage of specialisation. Thus specialisation leads to efficiency in accounting work.
(v) Helpful in decision making: Subsidiary books provide complete details about every type of transactions separately. Hence the management can use the information as the basis for deciding its future actions.
(vi) Prevents errors and frauds: Internal check becomes more effective as the work can be divided in such a manner that the work of one person is automatically checked by another person. With the use of internal check, the possibility of occurrence of errors or fraud may be avoided or minimised.
(vii) Availability of requisite information at a glance : The maintenance of subsidiary books helps in obtaining the necessary information at a glance.