At best, it could slow down a refund. At worst, it could unexpectedly increase a tax bill. And in the longer term, a name and ID number discrepancy could prevent your wages from being posted correctly to your Social Security record, which could mean you wouldn't get all the federal retirement benefits to which you're entitled.

That's why it's critical to make sure you:

1. Have a Social Security number for everyone listed on a return -- you, your spouse, children and any other dependents you claim.

2. Enter those numbers correctly and, if you're filing on paper, legibly.

Add your numbers

Before identity theft was such a problem, taxpayer Social Security numbers were preprinted on tax packages the Internal Revenue Service mailed out each year. That practice stopped years ago. The privacy enhancement, however, produced other problems.

The IRS found that removing the personal information also meant that some taxpayers forgot to enter their identification numbers on their tax returns.

So it's now up to you to fill in your Social Security number as well as any other numbers required on your return and associated schedules.

Marital mismatches

These numbers are particularly important for the recently married or divorced.

The IRS urges newlyweds, when the bride takes the husband's surname, to let the Social Security Administration, or SSA, know about the name change. If the couple files a joint tax return with her new name, IRS computers will not be able to match the new name with the number until the SSA is notified.

Similarly, the IRS warns, a woman who reassumes her maiden name after a divorce needs to make that change known to Social Security officials.

And if you pay alimony, you'll need your ex's tax ID number. The recipient's Social Security number must be included on the return of the former spouse who's making the payments if he or she deducts the spousal support payments.

9 critical digits

Why such concern over nine digits? Because there are so many transactions -- income statements, savings account interest, retirement plan contributions -- that are keyed to this number.

The identification numbers also are vital when the IRS checks any tax credits you apply for, such as the popular child and additional child tax credits, as well as tax breaks for educational expenses and dependent care.

If your kids don't have their Social Security numbers yet, contact the Social Security Administration immediately to obtain them. If you need to let Uncle Sam know of a name change, file Form SS-5, available on the agency's website. You also can request a form by calling (800) 772-1213 or pick one up at a local Social Security office. It usually takes two weeks to have the change verified.

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