US Residency Certification Apostille- Form 6166
Many U.S. treaty partners require the IRS to certify that the person claiming treaty benefits is a resident of the United States for federal tax purposes. The IRS provides this US residency certification on Form 6166, a letter of U.S. residency certification. Form 6166 is a letter printed on U.S. Department of Treasury stationery certifying that the individuals or entities listed are residents of the United States for purposes of the income tax laws of the United States. You may use this form to claim income tax treaty benefits and certain other tax benefits in foreign countries.