Form W-9 is a tax form used in the United States to provide a taxpayer identification number (TIN) to a requesting person or entity. It is commonly used by businesses to request the TIN of their vendors, contractors, or other payees.

Form W-9 is used to request the taxpayer identification number (TIN) of an individual or business. The TIN is typically the Social Security number (SSN) of an individual or the employer identification number (EIN) of a business. The TIN is used by the Internal Revenue Service (IRS) to identify taxpayers and track their income and tax liabilities.

Form W-9 must be completed and returned to the requesting person or entity in order for them to properly report the payment made to the payee. The payee is responsible for ensuring that their TIN is correct and for providing a correct TIN to the requesting person or entity.

Form W-9 is also used by individuals and businesses to certify their TIN and to confirm that they are not subject to backup withholding, which is a tax withheld from certain types of payments made to individuals and businesses.