Perhaps the most important amendment in American history was the 14th Amendment. Ratified in June 1868, it fundamentally reordered our system of federalism, adding new restrictions to state power. We best know the 14th Amendment through two of its clauses, one which says the state cannot deprive a person of life, liberty, or property without “due process of law” and another which guarantees to all persons the equal protection of the laws.

Debates regarding abortion, marriage, and segregation all have involved this amendment. And through its “incorporation” of the Bill of Rights at the state level, the 14th Amendment also has been part of most Supreme Court cases about free speech, freedom of the press, religious liberty, and the death penalty.

This Friday marks the 150th anniversary of the Supreme Court’s first 14th Amendment decision — the Slaughterhouse Cases, handed down in 1873. We’ve been reading the amendment wrongly ever since.

The case concerned the constitutionality of a monopoly granted by the Louisiana legislature to a slaughterhouse in the New Orleans area. This legal monopoly required all butchers in the area to use that slaughterhouse and to construct no others. The Supreme Court, though, didn’t focus on the due process and equal protection clauses that have since come to dominate our 14th Amendment (and general) judicial discourse. Instead, the justices focused on the privileges or immunities clause, which reads: “No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States.” A lot of people have never heard of this clause. Lawyers very rarely assert it and even less rarely win in court on the basis of it.

The privileges or immunities clause receives little attention because the Slaughterhouse Court gutted it from the start. As the Slaughterhouse dissent made clear, the 14th Amendment sought to fill a gap in the Constitution’s protection of fundamental individual rights. At that point, the Bill of Rights only applied to the national government, while the states had only a few constitutional limitations on their power.

By the end of the Civil War, the capacity of states to oppress persons under their jurisdiction, especially the newly freed slaves, was glaringly in view. The 14th Amendment was passed to create new restrictions on the states in service of individual rights. The due process clause would provide procedural protections of a fair trial before the state could punish a citizen; the equal protection clause would ensure that states afforded the protection of the laws to all, not just a favored group; and the privileges or immunities clause was meant to provide substantive protection to life, liberty, and property against any state actions seeking to infringe those privileges or immunities.

Taken together, the amendment provided a complete shield of protection against state oppression not previously known in the Constitution. “That amendment,” the Slaughterhouse dissent eloquently argued, “was intended to give practical effect to the declaration of 1776 of inalienable rights, rights which are the gift of the Creator, which the law does not confer, but only recognizes.”

That is, until the majority in Slaughterhouse wrongly interpreted the privileges or immunities clause. The majority made a distinction between two sets of privileges or immunities. One set, covered by the amendment, were those held by U.S. citizens as U.S. citizens. The other set, not covered by the amendment, consisted of those held by state citizens as state citizens. The court then classified all fundamental rights (life, liberty, property, etc.) as privileges or immunities of state citizenship. Those left to U.S. citizenship were passingly small and basically those already protected by the Constitution by earlier provisions.

The privileges or immunities clause has never been revived to its original, prominent, and crucial place within the 14th Amendment. Instead, we have stretched to deformity the due process clause, giving it a “substantive” meaning as well as its original procedural meaning so that it can carry the weight its neglected brother was denied.

On this 150th anniversary, we should remember the Slaughterhouse Cases for what it was: an egregious mistake in which the court set a crucial part of the Constitution on the wrong path — a path we remain on to this day. Perhaps one day, a court might restore the clause to its original intention (Justice Clarence Thomas is certainly trying). That restoration would be the best way to remember the 14th Amendment and the protections it provides.

Adam Carrington is an associate professor of politics at Hillsdale College.