According to its website, the Boston Public Library has a collection of more than 23.7 million items, which makes it one of the largest municipal public library systems in the United States. The vast majority of the collection – over 22.7 million volumes — is held in the Central Branch research stacks. Between July 2012 and June 2013, the annual circulation of the BPL was 3.69 million. Because of the strength and importance of its research collection, the Boston Public Library is a member of the Association of Research Libraries (ARL), a not-for-profit organization comprising the research libraries of North America. The New York Public Library is the only other public library that is a member of the ARL, and it also has a private endowment. The library has established collections of distinction, based on the collection's depth and breadth, including subjects such as Boston history, the Civil War, Irish history, etc. In addition, the library is both a federal and state depository of government documents.