Form W-9 is how a US vendor certifies its legal name, federal tax classification (individual, partnership, C or S corporation, LLC and how it’s taxed) and its taxpayer identification number. You keep it on file; you don’t send it to the IRS. It tells you whether the vendor gets a 1099 and which TIN to report.
Why it matters
The entity type decides reportability (most corporations are exempt), and the TIN decides whether you must backup-withhold. Collecting it at vendor setup is far easier than in January.
Worked example
A new drywall sub returns a W-9 checking “LLC, taxed as a partnership”. It’s reportable for services, so its labor payments count toward the 1099-NEC threshold (illustrative).
Common mistakes
- Accepting an incomplete or unsigned form.
- Not updating the vendor when its name or entity changes.
How os.construction handles it
In the accounting preview, each vendor’s accounting profile holds W-9 status and an encrypted, masked TIN, and a missing W-9 can hold payment. See Accounts payable.