A basic rule of double-entry accounting is that for every credit there must be an equal debit amount. From this concept, one can say that the sum of all debits must equal the sum of all credits in the accounting system. If debits do not equal credits, then an error has been made. The trial balance is a tool for detecting such errors.

The trial balance is calculated by summing the balances of all the ledger accounts. The account balances are used because the balance summarizes the net effect of all of the debits and credits in an account. To calculate the trial balance, construct a table in the following format:

In the above trial balance, the balances of Accounts 1, 2, and 3 are net debits, and the balances of Accounts 4, 5, and 6 are net credits. The totals of the debits and credits should be equal; if they are not, then an error was made somewhere in the accounting process. Some common errors include the following:

1. Error in totaling the columns - make sure that the trial balance columns were summed properly.

2. Error in transferring account balances to proper trial balance columns - make sure that debit and credit account balances are in the appropriate debit and credit columns of the trial balance calculation. Check for reversed digits and misplaced decimal points.

3. Omission of an account - an account may be missing in the trial balance calculation.

4. Error in account balance - an error may have been made in the calculation of a ledger account balance.

5. Error in posting a journal entry - a journal entry may not have been posted properly to the general ledger.

6. Error in recording a transaction in the journal - for example, making an error in a debit or credit, or failing to enter a debit or credit.

In general, the most effective way to isolate an error is to work backward from the trial balance itself to the initial journal entry, as outlined in the above list.

Note that a balanced trial balance does not guarantee that there are no errors. An error of omission could have been made in which a transaction was not recorded, a journal entry could have been posted to the wrong ledger account, or a debit and credit could have been transposed. Such errors are not caught by the trial balance.