What is an ad valorem tax?A tax based on the value of the articles being taxed.
What are direct taxes?A tax on persons (i.e., a capitation or head tax) or a tax on land. If Congress imposes a direct tax, the Constitution requires that it be apportioned among the states on the basis of population.
What is a duty?A tax on imports. Congress has the power to impose taxes on imports. The Constitution requires that taxes on imports be geographically uniform.
What is an excise?A tax on a good or service. The Constitution requires that federal excise taxes be geographically uniform.

What are exports?Domestic goods that are sold abroad. The Constitution prohibits taxes on exports.
What are imports?Goods brought into the United States from abroad. Only the federal government is authorized to tax imports.
What is intergovernmental tax immunity?The doctrine, traced back to McCulloch v. Maryland (1819), that the state governments cannot tax the federal government and the federal government cannot tax the states.
What is the Sixteenth Amendment?A constitutional provision, ratified in 1913, that authorizes Congress to enact a tax on income from whatever source derived.

What did the Supreme Court hold in United States v. Butler?The Supreme Court found the Agricultural Adjustment Act unconstitutional because it attempted to regulate and control agricultural production, an area reserved to the states. Even though Congress has the power to tax and appropriate funds, in this case those activities were ''but means to an unconstitutional end,'' and were therefore in violation of powers reserved by the states.
What did the Supreme Court hold in Pollock v. Farmers' Loan &Trust Co. (1895)?The Court ruled that the income taxes imposed by the Income Tax Act of 1894 were effectively direct taxes and, therefore, unconstitutional because Congress did not apportion them among states. This holding was superseded by the 16th Amendment (allowing Congress to impose income taxes without apportionment).
What did the Supreme Court hold in United States v. United States Shoe Corporation (1998)?The Court held that the Harbor Maintenance Tax (HMT), part of a federal law for the maintenance and development of harbors for shipping, violated the Export Clause which prohibits any tax on exports from the U.S. The Court held the HMT was a tax rather than a permissible user fee.
What did the Supreme Court hold in South Carolina v. Baker (1988)?The Court ruled that a federal tax on the interest earned on state bonds does not violate the intergovernmental tax immunity doctrine. The federal tax did not discriminate between interest earned on private and public bonds and therefore did not tax state government activity as such.

What did the Supreme Court hold in Davis v. Michigan Department of Treasury (1989)?The Court held that Michigan violated the intergovernmental tax immunity doctrine when it exempted state and local government pensions from taxation but levied taxes on federal government pensions.
What did the Supreme Court hold in McCray v. U.S. (1904)?The Court upheld the excise tax on oleomargarine established by the Oleomargarine Act of 1902. Congress has broad authority to impose taxes and Court should not question the purpose and motive behind particular taxes.
What did the Supreme Court hold in Bailey v. Drexel Furniture Co. (1922)?The Court found the 1919 Child Labor Tax Law unconstitutional as an improper attempt to penalize employers using child labor. The Court indicated that the tax imposed by the statute was actually a penalty in disguise.
What did the Supreme Court hold in Steward Machine v. Davis (1938)?The Court upheld the unemployment compensation provisions of the Social Security Act of 1935 which established a national taxing structure designed to induce states to adopt laws for funding and payment of unemployment compensation. The decision signaled the Court's acceptance of a broad interpretation of Congressional power to influence state laws.

What did the Supreme Court hold in South Dakota v. Dole (1987)?The Court upheld the constitutionality of a federal statute that withheld a percentage of federal funds from states that did not raise their legal drinking age to 21. The federal government may use its spending power to encourage states to legislate.
What did the Supreme Court hold in National Federation of Independent Business v. Sebelius (2012)?This landmark decision addressed a number of constitutional arguments. With respect to the claim that Obamacare's "shared responsibility payment" is a penalty rather than a tax, a 5-4 majority find the payment is a tax. However, a 7-2 majority holds that Congress may not threaten to withhold all state Medicaid funding to encourage Medicaid expansion.