Form 8879 - Practitioner PIN Method
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The IRS has a website containing information, guidelines and FAQs for preparers using the Practitioner PIN Method for Forms 1040 and 4868 Modernized e-File (MeF):
 
IRS added Forms 1040, 4868, and 21 1040-related tax returns to the Modernized e-File platform in February 2010. The Practitioner PIN option for electronically signing Forms 1040 and 4868 through the MeF platform is provided below. The requirements for the Practitioner PIN are the same for both the current e-file system and the MeF system. However, there are slight differences in how information is transmitted or returned between the 2 platforms. "MeF info" is noted next to any question/answer that has MeF specific information.
 
The Practitioner PIN method is an additional signature method for taxpayers who use an Electronic Return Originator (ERO) to sign their return by entering a five-digit PIN. The PIN can be any five digits except all zeros.
Generally, a PIN is needed for each taxpayer when completing a married filing joint tax return. The Practitioner PIN method allows you to authorize the Electronic Return Originator to enter or generate your PIN. It is not necessary for taxpayers to provide their date of birth, prior year adjusted gross income or prior year PIN for authentication when using the Practitioner PIN method. 
 
Following this section is a list of General Q&A's for Electronic Signature (PIN) Methods.