The Indigo Book

The Indigo Book: An Open and Compatible Implementation of A Uniform System of Citation (formerly Baby Blue's Manual of Legal Citation) is a free content version of the Bluebook system of legal citation. Founded by New York University professor Christopher Jon Sprigman, authored collectively by Sprigman and a group of NYU law students, and published by Public.Resource.Org, it is an adaptation based on the 10th edition of the Bluebook as published by the Harvard Law Review Association in 1958, which had entered the public domain in the United States because its copyright had expired due to non-renewal.

The project was inspired by correspondence between Public.Resource.Org's founder Carl Malamud and a Nagoya University academic, who was threatened by lawyers representing the HLRA over plans to incorporate the Bluebook system into the open source citation management program Zotero. Sprigman has argued that the system of citation expressed in the Bluebook was effectively public domain because its mandated usage in courts made it an "edict of government", and because, barring trivial changes, the then-current 19th edition was nearly identical to the public domain 10th edition. Sprigman stated that the project's main goal was to allow the Bluebook's system of citation to be widely available at no cost, and allow others to collaborate on it under an open-source model.

The Indigo Book is an unofficial substitute to the official Bluebook and is not endorsed by the Harvard Law Review Association; in December 2015, the project faced legal threats over its original name, Baby Blue's, which lawyers representing the HLRA felt was too similar to the Bluebook trademark. These threats led to the renaming of the guide to The Indigo Book in March 2016.