This question refers to 1040 tax form for 2015 on http://www.irs.gov/pub/irs-dft/f1040--dft.pdf
Am I understanding this correctly?:
Capital gains (on line 13) is part of total income (on line 22). So capital gains is often taxed twice:
1) taxed as captial gains (which can be taxed at 0%, 15%, or 20%) 2) capital gains is added to total income
If total taxable income is greater than 0, then capital gains is taxed as capital gains and again as income.
If total taxable income is 0, then both capital gains and income are taxed at a 0%.