The federal government collects revenue from a variety of sources. The largest source is taxes on individuals’ income and payroll taxes, which together make up four-fifths of federal revenue. Individual income taxes are assessed against wage income and are progressive: that is, the tax rate someone pays increases as their income increases. Payroll taxes are a flat percentage of wage income. They are used to fund Social Security and portions of Medicare. Taxes on corporate profits, estate and gift taxes, excise taxes (a tax levied on a good or service), customs duties, and numerous other taxes comprise the remainder.