This is a difficult concept that troubles even law school students when they take and study constitutional law. However, it is a vitally important concept to understanding the U.S. system of constitutional law.

Let’s use the example of the First Amendment freedom of speech to explain how this works. The First Amendment prohibits the federal government from punishing individuals’ freedom of speech under many circumstances. However, in the nineteenth century the U.S. Supreme Court determined that the entire Bill of Rights only pro tected individuals from the federal government. This meant that the First Amendment freedom of speech only protected individuals from invasions by federal officials. If a state or local police officer violated an individual’s free-speech rights, he or she had to assert a constitutional claim under the state constitution.

That changed with the U.S. Supreme Court’s interpretation of the First and Fourteenth Amendments in the early twentieth century. Remember that the due-process clause provides: “nor shall any state deprive any person of life, liberty or property without due process of law.” The U.S. Supreme Court determined that “freedom of speech” was part of the “liberty” mentioned in the due-process clause of the Fourteenth Amendment.

In other words, the due-process clause provides that no state shall deprive any person of liberty, which includes freedom of speech. Throughout most of the twentieth century, the U.S. Supreme Court, through the process of selective incorporation and the due-process clause, extended the bulk of the Bill of Rights to the states.