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Congress has enacted a series of "patches" in recent years to spare huge numbers of people from getting hit.
An estimated 4.8 million Americans will be affected by the AMT in 2012, according to the Tax Policy Center in Washington. Without a patch, however, more than 31 million families, or more than one-third of all U.S. taxpayers, would be snagged.
The most recent patch covered 2010 and 2011. There is no fix yet for 2012, but the adjustments often come late in the year when squabbling lawmakers acquiesce on issues with broad consensus.