Although this number is probably something you learned by heart in elementary school, if you have any doubt, keep your card handy. "Your Social Security number is used as your taxpayer identification when filing a return," says H&R Block (HRB - Get Report) senior tax preparer Riley Holmes. "Social Security cards are used to verify the spelling of the individual's name as well as their SSN itself so the return matches their IRS master file and processes smoothly," says Jackson Hewitt Chief Tax Officer Mark Steber. If you need a cheat sheet and can't find your card, your Social Security number will also appear on the W-2 from your employer. Salary documentation (W-2)
"Your W-2 is the key form, and you'll need one from each employer you worked for during the past year," says Paul Golden, spokesman for the National Endowment for Financial Education. "Your W-2 shows how much money you made, how much income tax was withheld, Social Security and Medicare taxes paid and any benefit contributions -- retirement plans, medical accounts and child care reimbursement plans," Golden says.
If you held more than one job during the past year, making sure you have W-2s from all your employers is imperative, Steber says. "If you leave any income items off your return and IRS catches it -- and they will -- then you will face penalties, interest and other issues," Steber says.