A tax consists of a rate and a base. Because income is the base for the income tax, a central question is: What constitutes income? Different theoretical concepts of income exist in economics, accounting, and taxation. The base of income to which the federal income tax rate structure applies is taxable income as constitutionally and statutorily defined. Thus, the concept of taxable income is grounded in theory and modified by political dynamics and administrative concerns.

From its modern introduction in 1913, the rate structure for the individual income tax has been progressive, meaning that tax rates graduate upward as the base of taxable income increases. Different tax rates apply to ranges of income, called brackets. Over time, the number of brackets and tax rates that apply to them have varied greatly. The tax rate applied to the last dollar of taxable income earned...