It's worth remembering that the tax code is progressive, so your marginal tax rate is the top tax rate you pay — the rate you would pay on an additional dollar of income. (fidelity.com)
The tax credits «phase - out» within certain income ranges, with their value dropping with each additional dollar of income. (educationnext.org)
A marginal tax rate is the amount of tax paid on an additional dollar of income. (investopedia.com)