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The Friedberg numbering system was developed by Robert Friedberg, and it is the numbering system used in Paper Money of the United States 20th Edition (released May 15, 2013) by Arthur L. and Ira S. Friedberg. It is considered the standard, universally accepted method of describing and cataloging United States paper money. These catalog numbers are used as a shorthand method of systematically identifying a particular note based on its design, series, and signature combination. Each Friedberg number represents a unique combination of these factors.