The phrase "ex rel" is an abbreviated version of the Latin phrase ex relatione, which means “by the relation of.” As a legal term, this phrase is used to indicate that a case is being brought by a plaintiff at the request of or in the interest of another party. The phrase is the procedural equivalent of “on behalf of” or “for the use of.”
The individual with a vested interest in the case is called the relator. A typical ex rel case would be filed by a representative of the government, acting on the behalf of a private citizen as the relator. The case is based on information provided by or that pertains to the relator.
In the United States, ex rel cases are commonly filed by the state’s Attorney General or, at the federal level, by the U.S. Department of Justice. When citing such a case, the plaintiff is listed first, followed by the phrase "ex rel," and then the name of the relator. For example, the 1938 U.S. Supreme Court case Missouri ex rel. Gaines v. Canada was filed by the State of Missouri on behalf of the individual Lloyd L. Gaines.
Ex rel cases can be civil or criminal. They usually are brought when the interests of the individual closely align with the interests of the government or the public. Cases involving whistleblowing and civil rights cases are often filed in this manner.
Although it is less common, ex rel cases can also be filed by individuals or groups on behalf of another individual. These cases typically are filed on the behalf of a relative or someone who has a contractual interest in the case. For example, in a 2005 appeal made to the U.S. Supreme Court, Schiavo ex rel. Schindler v. Schiavo, the plaintiff, Terry Schiavo, was listed as incapacitated, and the case was filed at the instigation of and for the benefits of her parents, Robert and Mary Schindler.