Fontaine v. McNab was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner who was being held in a federal prison. The case arose when a prisoner, Fontaine, was held in a federal prison in Louisiana. Fontaine sought a writ of habeas corpus from the state court, claiming that he was being held in violation of the Constitution. The state court granted the writ, and the federal government appealed the decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The Court reasoned that the writ of habeas corpus was a federal remedy, and that the state court did not have the power to interfere with the federal government's authority to imprison individuals. The Court also noted that the writ of habeas corpus was a remedy that could only be used to challenge the legality of a person's detention, and not to challenge the conditions of the detention. The Court's decision in Fontaine v. McNab established that state courts do not have the authority to issue writs of habeas corpus to prisoners held in federal prisons. This decision has been cited in numerous subsequent cases, and has been used to support the principle that state courts cannot interfere with the federal government's authority to imprison individuals.

In Fontaine v. McNab, the Supreme Court was asked to decide whether a contract between two individuals that involved the sale of land in Louisiana was valid under state law. The majority opinion held that it was not, as the contract did not comply with certain requirements set forth by Louisiana's Civil Code. Justice Field dissented from this decision and argued that while there were some technical deficiencies in the agreement, they should be overlooked due to its substantial compliance with all other aspects of state law. He further noted that if such minor errors were enough to invalidate contracts then many agreements would be rendered unenforceable and parties could easily avoid their obligations without consequence. As such, he concluded that courts should take into account both formalities and substance when determining whether an agreement is legally binding or not.