GREENVILLE, S.C. -- To you, your Social Security number is a set of nine digits you've probably had memorized since you moved out of your parents' home.

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To businesses, it's a way to keep tabs on customers' accounts, check credit histories and develop marketing lists.

To a thief, it's nine digits of gold.

"With a Social Security number, you can find out everything about somebody very quickly," Elizabeth Rollyson, of Compass of Carolinas, told WYFF-TV in Greenville, S.C.

Rollyson said the number is a perfect tool for tracking you.

"It makes it easier for them to track you down, anybody to track you down, and that means marketers, all kinds of people," she said.

While many businesses may ask for your number, the Social Security Administration says most aren't entitled to it by law.

Government entities, such as the department of motor vehicles, welfare, or tax departments, can require the number legally.

Any institution that deals with taxes, such as banks, brokerages and employers, can also require your SSN.

Most other businesses don't have the legal authority to require your number.

However, any business that asks for your SSN can refuse you service or products if you don't give it to them, even if they aren't legally entitled to it.

"A lady went to take her cat to the vet, and they asked for her Social Security number, and almost didn't give her services because she wouldn't give her Social Security number," Hollyson said.

Hollyson said the best policy is to ask questions.

"Ask what their privacy policies are. Ask if you can have your name removed from a list. If you're giving them your Social Security number, are they going to include your name on a marketing list?" she said.

The SSA says you should also ask:

Why your number is needed?
How your number will be used?
What happens if you refuse?
What law requires you to give your number?

Privacy experts say your SSN is the keystone to your financial life, and thieves will work long and hard to obtain it.

Hollyson said your best defense is to give the number out as little as possible and check your credit history every year for any anomalies.

On the Net: Social Security: Your Number And Card