Justice Harlan Fiske Stone wrote a footnote in his opinion for the Court in United States v. Carolene Products Co. that has become known as the famous “Footnote Four.” The case concerned the constitutionality of the Filled Milk Act of 1923, which prohibits the shipment in interstate commerce of skimmed milk made up of any fat or oil other than skimmed milk. The Court ruled 6–1 in favor of the constitutionality of the law.

The case is not considered important except for the footnote, which provided that there should be different degrees of judicial scrutiny for different types of laws. Laws that impact economic matters should be reviewed under the deferential rational basis test, which provides that a law is constitutional if Congress had a reasonable, or rational, basis for passing the law. However, other types of law that impact fundamental individual freedoms, such as the freedom of speech or the right to vote, must pass a higher degree of judicial review known as strict scrutiny.