The United States Congressional Serial Set is commonly referred to as the Serial Set. Publication of Serial Set volumes began in 1817 (15th Congress). The consecutively-numbered books compile congressional publications for physical distribution. The term Serial Set is used because each volume in the series has a unique number that is useful for locating an item within the series. Congressional publications compiled within the Serial Set include Congressional Reports, House Documents and Senate Documents. Executive-branch materials were published in the Serial Set during the late nineteenth and early twentieth centuries. Congressional publications prior to 1817 may be found in the American State Papers.