Hearings were not published generally until the latter part of the 19th century, except some early hearings (generally of special investigative committees) were published in the series that are part of the Serial Set. Published hearings did not become available for purchase from the United States Government Printing Office until 1924 and were not distributed to depository libraries until 1938. Unlike the documents and reports that are compiled in the Serial Set "hearings do not constitute a real series" although in the modern era a trend toward uniformity of numbering has resulted in all Senate hearings and prints for each Congressional Session (commencing with the 98th Congress in 1983) being assigned a unique numerical designation (in the style of what one scholar dubbed a "combination code") published on the cover and title page (e.g. S. HRG. 110-113; S. PRT. 110-13). A growing number of House Committees are assigning numerical or alphabetical designations for their publications (e.g. 110-35, 110-AA).