To view the filing instructions which include attachment requirements, go to our Print Filing Instructions Online and Desktop FAQ.
Per IRS Publication 17 Your Federal Income Tax (For Individuals), page 13:
Depending on the form you file and the items reported on your return, you may have to complete additional schedules and forms and attach them to your paper return.
TIP: You may be able to file a paperless return using IRS e-file. There's nothing to attach or mail, not even your Forms W-2. See Why Should I File Electronically, earlier.
Form W-2. Form W-2 is a statement from your employer of wages and other compensation paid to you and taxes withheld from your pay. You should have a Form W-2 from each employer. If you file a paper return, be sure to attach a copy of Form W-2 in the place indicated on your return. For more information, see Form W-2 in chapter 4.
Form 1099-R. If you received a Form 1099-R showing federal income tax withheld, and you file a paper return, attach a copy of that form in the place indicated on your return.
Form 1040 or 1040-SR If you file a paper return, attach any forms and schedules behind Form 1040 or 1040-SR in order of the “Attachment Sequence No.” shown in the upper right corner of the form or schedule. Then, arrange all other statements or attachments in the same order as the forms and schedules they relate to and attach them last. Don't attach items unless required to do so.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.