A W-9 is the form on which a contractor supplies their legal name, business structure and taxpayer identification number.
Why it is needed
A business paying a contractor above a threshold in a year must report those payments to the tax authority and send the contractor a copy. That report requires the identification number, which comes from the W-9.
Collect it before paying, not at year end
This is the whole of the practical advice.
A contractor who has been paid and has moved on has no reason to respond. Collected before the first payment, it is a condition of being paid and is supplied immediately.
The alternative is chasing missing details in January for work done the previous March, which is where the penalties for late or incomplete reporting are incurred.
What it establishes
The legal name and structure, which is frequently not the trading name on the invoice.
The identification number, and whether the payee is exempt from reporting.
Related paperwork
For a subcontractor, the W-9 sits alongside a current certificate of insurance and any licence required for the trade. All three are collected at the same point, before work starts.
Whether the person should be a contractor at all is a separate and prior question — see employee or contractor.