WASHINGTON — A primer on the alternative minimum tax (AMT):

Q: What is the AMT?

A: It's a parallel tax to the regular income tax. It was created by Congress in 1969 to close tax shelters that the richest Americans — those with incomes exceeding $200,000 then — exploited to avoid taxation.

Q: Why is the AMT a problem now?

A: The AMT never was indexed to inflation. That means what was considered sky-high taxable income in 1969 is middle-class income today in expensive areas. Congress typically passes a one-year fix to limit its bite. But Democrats and Republicans this year are at odds over how to fix it.

Q: How do I know if I'm vulnerable?

A: You must calculate your income taxes two ways — once the regular way, once under AMT rules.

Q: Who is most at risk for the AMT?

A: A tax filer taking lots of deductions. That means married couples with lots of children, high-income filers who deduct sales taxes and interest on home loans, and particularly people in states with high property taxes that are deducted from federal income taxes.

Q: Which states or places have the biggest AMT hit because of high property taxes?

A: New Jersey, New York, Connecticut, the District of Columbia, Maryland and California historically have the highest percentage of tax returns subject to the AMT.

Q: Which states have the lowest percentage of filers subject to the AMT?

A: Lowest is South Dakota, followed by Mississippi, Tennessee, Alaska and Alabama.

Q: If there isn't an AMT patch, taxpayers in which states could face the most in new taxes?

A: In order, California, New York, Texas, New Jersey, Pennsylvania, Illinois, Florida, Michigan, Ohio and Massachusetts, according to the House Ways and Means Committee.

Q: What does the fix, or patch, do?

A: It essentially freezes the AMT at a point where it hits about 4 million tax filers, instead of 23 million.

Q: So why not patch it?

A: The AMT is projected to haul in more than $850 billion in tax revenues over the next decade. A lot of official assumptions about future government finance assume that the AMT will be collected. To repeal the AMT and offset the massive hit that would deliver to the federal budget, Congress would have to cut spending, increase taxes or both.