Taxpayer Identification Number (TIN)

A Taxpayer Identification Number (TIN) is any identifier the IRS uses to track a taxpayer. For subcontractors, the TIN is typically either a Social Security Number (SSN) for sole proprietors or an Employer Identification Number (EIN) for businesses. The TIN is collected on Form W-9 and used to file Form 1099-NEC at year-end. An incorrect or missing TIN can trigger backup withholding, IRS matching notices (CP2100), and penalties. Verifying the TIN against IRS records during onboarding is a best practice for any GC that files more than a few 1099s.