| When you file an amended tax return, the additional amount that you owe will be calculated on this form. You will then enclose a check along with the 1040X Amended tax return and mail to the IRS Service Center where you originally filed your tax return. By filing an amended tax return, and enclosing a check to satisfy the additonal tax liability, you would avoid having to be pay the late filing penalties and interest on the underpayment! | 
