The AMT is an alternative tax system - taxpayers owe whichever is greater. If your liability in AMT (known as your "Tentative Minimum Tax") exceeds your regular tax liability, then the difference becomes your AMT liability on your tax return. This calculation is done BEFORE factoring in tax credits such as the EITC or Child Tax Credit, so what the line on this graph shows is your Tentative Minimum Tax liability, minus any tax credits. In most situations, the ordinary tax line will be above the AMT line - if the AMT is on top, this represents someone who owes AMT.