U.S. Supreme Court justices on both sides in the landmark D.C. v. Heller gun rights case resorted to original documents in making their case about the meaning of the Second Amendment. But they used a little-known digital resource to get there, a project whose mission is to digitize thousands of Founding-era documents that shed light on the original meaning of the Constitution.

The Constitutional Sources Project, which launched publicly last September, has digitized and made freely available online more than 11,000 historical documents relating to the Constitution and the amendments. Among them are at least 20 documents cited by majority and dissenting opinions in Heller, says the project’s co-founder and executive director Lorianne Updike.