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 Rectification Of One-sided Errors Located Before Preparation Of Trial Balance
One-sided errors are those errors which affect only one side of an account. Wrong totaling of subsidiary books, posting a wrong amount, posting on the wrong side are some of the examples of one-sided errors. Since two accounts are not involved in these errors, journal entry can not be passed for rectifying such errors. The one-sided error is rectified by making an additional posting on the affected side of the ledger account.

* Rectification Of Two-sided Errors Located Before Preparation Of Trial Balance
The errors that affect two or more accounts are called two-sided errors. Correction of such two-sided errors needs to make rectification journal entries since such errors involve two or more accounts. Therefore, one account is debited and another account is credited to rectify two-sided errors. The rules of debit and credit are applied to rectify these errors.
The following three steps are taken to rectify the two-sided errors.
a. Identify correct entry
b. Rewrite wrong entry
c. Find rectifying entry by making adjustment of correct entry and wrong entry
 

read more at 

https://accountlearning.blogspot.in/2010/07/rectification-of-accounting-errors-and.html

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