Single Entry System :
In olden days businessmen used to keep the record of the transactions in a simple diary or a small book. They were unaware of the book-keeping system. This made impossible for them to find the correct financial position of the business. They were unable to find the correct profit or loss of the business. Only some personal accounts and real accounts were maintained in the books that too without applying any rules and principles. This type of system was called as single entry system.
Single entry is an unfit and incorrect form of Book-keeping. In this system only cash account and personal accounts are maintained. We cannot ascertain clear correct profit or loss under this system. So, it is an incomplete system and not in general use. Some small enterprenuers still use this system. For example : Shop-keepers, hawkers, etc.
Double Entry Book-Keeping :
Due to many drawbacks in single entry system the double entry system was introduced. Double Entry System is a system in which every business transaction has minimum two parties involved.
One is the giver and the other is the receiver. In some transactions two things are involved.
Something comes in the business while something goes out of the business.
Similarly there are some Incomes while some are expenses.
Some are profits while some are losses.
So it becomes clear that in every transaction there are at least two accounts effected.
One is debited and the other is credited. Recording 'double' or 'two-fold' effect of each transaction is known as Double entry. In double entry system, every debit has equal credit. The amount of debit is always equal to the amount of credit, no matter how many number of accounts are debited or credited.
Accountancy Concepts
Entity Concept :
The word entity means existence. In accounting sense a business is treated as separate 'entity' or a separate 'unit' from its owners. Business is an artificial person created by law. It has a separate legal entity. In accounting sense the owner of the business is also treated as a third party i.e. a creditor of the business. So the individual transactions of the owner are not recorded in the business books of accounts. Only business transactions are recorded in the business books. It is necessary to know the actual profit / loss of the business.
For example : If Ram is the owner of a business named by 'Ishwar Enterprises'. Then Ram is a separate entity and is a natural person account, because he is created by nature while 'Ishwar Enterprises' is a separate entity and is an artificial personal account.
Going concern OR Continuity Concept :
In accounting sense it is supposed that the business has a perpetual succession or a very long life. It will continue to exist for an unlimited period. Even though the businessman dies his successors will continue the business. This confirms that businessman may end but the business never ends. No businessman starts a business with the intention to close it down after a specified period. So, the financial statements of each year are prepared on this concept.
For example : Business of Tata group was started by Jamshedji Tata. His successors are still continuing the business. Similarly Bajaj Group of companies was started by Jamnalal Bajaj. Now his successors are running the business..
Realisation Concept OR Revenue Recognition Concept :
Revenue is the gross earning. It is realised when goods and services are transferred to a customer for cash or for some other asset or service or for a promise to pay cash or other asset in future. Unless revenue is treated as realised, it should not be credited in as an income earned by the business.
For example : Goods sold on credit to a customer Rs.10,000 but only Rs.9,000 have been received yet. Here the remaining Rs.1,000 is not treated as revenue as the income is not yet generated.
Accounting Period Concept :
It is assumed that the life of any business is indefinite. It is necessary for the business to know the exact financial position on a particular date. They have to know the profit & loss at the end of the year. For this purpose generally a period of twelve months is set. This period is known as Accounting Period. At the end of each financial period, final accounts are prepared to know the profit earned or loss incurred during the period. Even if the accounting period for external use is of one year, the management can fix the accounting period of much shorter interval i.e. 6 months or 3 months as required for the internal audit. In general financial statements are prepared for one year starting from 1st April of the current year and ending on 31st March of the next year.
Money Measurement Concept :
In accounting, all transactions are expressed in terms of money, not in terms of quantity. Transactions which cannot be measured in terms of money are not allowed in accounting. Transactions which have exchange value are considered may they be credit transaction, cash transaction or barter transaction.
For example : A business owns two buildings, one costs Rs.10,00,000 and the other costs Rs.5,00,000. While recording in asset side of the balance sheet the amount Rs.15,00,000 is to be written instead of writing two buildings.
Similarly if the business owns three motor vehicles. One for Rs.50,000, second for Rs.45,000 and the third Rs.55,000, we have record in the books as 'Motor Vehicles Rs.1,50,000 and not as three vehicles.
This makes clear that every transaction is measured in terms of money no matter what the quantity is.
Conventions :
Commonly accepted rules that are used in accounting are conventions.
Conservatism :
This convention explains that expected profits are not to be considered but expected losses are to be considered. This is because we cannot judge the customers intention of paying the debts. But we are of the intention of payment of our dues.
For example : We are payable to a creditor Rs.10,000. We will pay it as and when cash is realized. We also have debtors of Rs.5,000. But we are not confident that we will realize the cash.
That is why Reserve for doubtful debts are made but Reserve for discount on creditors is not made. Similarly closing stock is valued at cost or market price whichever is less.
Consistency :
This convention makes clear that once any system or method is applied. It should be applied for consecutive many years, so that the company can anticipate the budgets with the help of the past records. Any method or policy should not be changed, unless and otherwise the conditions allow.
For example, There are different method of depreciation such as Fixed Instalment Method, Diminishing Balance Method, Annuity Method, Sinking Fund Investment Method, etc. Once a method is applied should be followed continuously every year. If it changes it will be very difficult to compare the results of different periods. There are four methods of Stock Valuation. They are FIFO, LIFO, Simple Average and Weighted Average. Once a method is applied it should be followed for consecutive many years.
Materiality :
In a business it is not possible to record every small thing. Therefore the importance of the thing is judged and recorded if it is worth recording. Materiality means sensiblility. It means to decide what is important and what is not. Items ascertaining financial condition and the calculation of profits earned or losses incurred should be recorded and exposed in financial statements.
For example : Hundred files are bought for Rs.10 each and a dozen of pencils are bought for Rs.2 each. Amount of each file or each pencil is not recorded but the lot amount is recorded in the books of accounts. So the amount recorded as 'Stationery' is (10 X 100) + (2 X 12) = Rs.1,024.
Types of Accounts
Personal Account :
Account recording transactions which affect a person or a firm is known as Personal Account. They are again divided into Natural, Artificial or Representative Personal Accounts.
Impersonal Account :
Account recording transactions which do not affect particular a person, but affect business in general, is known as impersonal account. Impersonal account is again divided into two. They are Real Account or Nominal Account.
Real Account :
Account of property or possession is called Real Account. For e.g. Goods A/c., Cash A/c., Bank A/c., Furniture A/c.
Nominal OR Fictitious :
All expenses or losses, incomes or gains include nominal account. For e.g. Rent A/c, Salaries A/c., Advertisement A/c.

RULES OF ACCOUNTS
There is one rule for Personal Account no matter it is Natural, Artificial or Representative.
But for Impersonal Accounts there are two different rules for Real Account and Nominal Account
Let us see those rules
PERSONAL ACCOUNT - Debit the Receiver
Credit the Giver
REAL ACCOUNT - Debit What Comes in
Credit What Goes Out
NOMINAL ACCOUNT - Debit all Expenses & Losses
Credit all Incomes & Gains