The Alternative Minimum Tax attempts to ensure that anyone who benefits from certain tax advantages pays at least a minimum amount of tax. The AMT provides an alternative set of rules for calculating your income tax. In general, these rules should determine the minimum amount of tax that someone with your income should be required to pay. If your regular tax falls below this minimum, you have to make up the difference by paying alternative minimum tax.
Since its creation in the 1960s, the Alternative Minimum Tax (AMT) has not been adjusted for inflation. Thus, Congress was forced to “patch” the AMT by raising the exemption amount to prevent middle class taxpayers from being hit by the tax as a result of inflation.
On January 2, 2013 the American Taxpayer Relief Act of 2012 finally indexed the income thresholds to inflation, preventing the necessity for an annual patch.