Why does the U.S. Internal Revenue Service (IRS) refer to the Certification of U.S. Residency for Tax Treaty Purposes as a form (Form 6166)?


a printed document with blank spaces for information to be inserted.

which that doesn't seem to match the description of a certificate.

Form 6166:

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(image source)

1 Answer 1


A form is a document which has a predefined structure (or "form") These letters have defined structure. They always say the same thing "This person is a resident of the USA for tax purposes". They always use the same standard headed paper.

Moreover, in older use a "form" is a "standardised legal document"

Finally, calling this a form gives consistent naming. In the IRS system everything is a "form". (Compare this to the UNIX system where "everything is a file")

  • Very interesting, thanks! Commented Jun 6, 2020 at 19:25
  • As to "certification", it's completing the form that does the certifying. If government issues a form that certifies something, that might be called a certificate. Commented Jun 6, 2020 at 20:05
  • For my money, as it were, the last paragraph is the only answer to this question.
    – phoog
    Commented Jun 7, 2020 at 8:20

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