In United States constitutional law, the penumbra includes a group of rights derived, by implication, from other rights explicitly protected in the Bill of Rights. These rights have been identified through a process of "reasoning-by-interpolation", where specific principles are recognized from "general idea[s]" that are explicitly expressed in other constitutional provisions. Although researchers have traced the origin of the term to the nineteenth century, the term first gained significant popular attention in 1965, when Justice William O. Douglas's majority opinion in Griswold v. Connecticut identified a right to privacy in the penumbra of the constitution.

William Orville Douglas was an American jurist who served as an associate justice of the Supreme Court of the United States from 1939 to 1975. Douglas was known for his strong progressive and civil libertarian views and is often cited as the U.S. Supreme Court's most liberal justice ever. Nominated by President Franklin D. Roosevelt in 1939, Douglas was confirmed at the age of 40, becoming one of the youngest justices appointed to the court. In 1975, Time called Douglas "the most doctrinaire and committed civil libertarian ever to sit on the court." He is the longest-serving justice in history, having served for 36 years and 211 days.