In Revenue Ruling 2008-29 [Internal Revenue Bulletin 2008-24], IRS discusses income tax withholding in nine different situations involving the payment of supplemental wages:

1. Commissions paid at fixed intervals with no regular wages paid to the employee
2. Commissions paid at fixed intervals in addition to regular wages paid at different intervals
3. Draws paid in connection with commissions
4. Commissions paid to the employee only when the accumulated commission credit of the employee reaches a specific numerical threshold
5. A signing bonus paid prior to the commencement of employment
6. Severance pay paid after the termination of employment
7. Lump sum payments of accumulated annual leave
8. Annual payments of vacation and sick leave
9. Sick pay paid at a different rate than regular pay