<p>Between 1880 and 1920, several inventors in St. Louis, Missouri, made pioneering contributions to the development of commercial adding machines. Inventions of Frank S. Baldwin, William S. Burroughs, the brothers William W. and Hubert H. Hopkins, and their associates did much to make mechanical arithmetic commonplace in U.S. business.</p>

Peggy Aldrich Kidwell, "The Adding Machine Fraternity at St. Louis: Creating a Center of Invention, 1880-1920", IEEE Annals of the History of Computing, vol. 22, no. , pp. 4-21, April-June 2000, doi:10.1109/85.841133