When to request a W-9
Request a Form W-9 from any vendor or contractor before issuing the first payment of the engagement. The W-9 establishes the payee's legal name, taxpayer identification number (SSN, EIN, or ITIN), federal tax classification (individual, C-corp, S-corp, partnership, LLC, trust), and exemption status from backup withholding. Without a valid W-9 you must withhold 24% of the payment as backup withholding under Section 3406.
Who provides a W-9
US persons (citizens, resident aliens, US partnerships and corporations, and US trusts) provide Form W-9. Non-US persons provide the W-8 series instead — W-8BEN for individuals, W-8BEN-E for entities, W-8ECI for income effectively connected with a US trade or business, and W-8IMY for intermediaries.
Why the LLC checkbox matters
The LLC line is the most common source of error on Form W-9. A single-member LLC owned by an individual is a "disregarded entity" and should check the individual box and provide the owner's SSN — not the LLC's EIN. A multi-member LLC checks "Partnership". An LLC that has elected S-corp or C-corp status checks "Limited liability company" and writes the appropriate tax classification (S or C) on the line.
TIN matching
Payers should run the IRS TIN Matching Service against W-9 data before filing 1099 forms. A name-TIN mismatch triggers a CP2100 notice, which obligates the payer to begin backup withholding within 30 days unless the payee corrects the discrepancy. Catching mismatches at onboarding avoids that administrative hassle.
Retention
Keep completed W-9 forms for at least four years after the last reportable payment. The W-9 is not filed with the IRS — it is a payer's recordkeeping document — but it must be available on request during an information return audit.
What it does not cover
Form W-9, Request for Taxpayer Identification Number and Certification does not stand alone — it lives inside a small ecosystem of supporting forms, schedules, and elections that together carry the full weight of a small-business return. Knowing what Form W-9, Request for Taxpayer Identification Number and Certification does not handle is just as important as knowing what it does. The instructions list the cross-references explicitly: items computed elsewhere (Schedule C net profit, Schedule SE self-employment tax, Form 4562 depreciation), items reported on a separate form (Form 8829 home office, Form 1099-NEC information returns), and items handled by an entirely different return entirely (Form 1120-S for S-corps, Form 1065 for partnerships). Drawing the boundary cleanly avoids double-counting and avoids leaving deductions on the table.