Contra account is an account that appears as a subtraction from another account on balance sheet of a company. For example, provision for bad debts is a contra account of accounts receivable. Accounts receivable is the main account with a debit normal balance and provision for doubtful debts is an account with credit normal balance which offsets the main account.
In an accounting system, ledger accounts are designed to contain similar classes of transactions. But for presentation purposes it is sometimes necessary to show net balance of two accounts. For example, there need to be separate accounts to hold the actual cost of property, plant and equipment and related accumulated depreciation. If we record depreciation related adjustments in the cost accounts we will lose key information about the original cost of the assets and accumulated depreciation. To avoid this loss of important data, we record actual cost and depreciation in separate ledger accounts. However, it will be meaningless to show the property plant and equipment cost account and the related accumulated depreciation account separately on the balance sheet. We need to show the net book value of the property plant and equipment which equals the cost of PPE offset by related accumulated depreciation which is a contra account.
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