Monroe Model K-121 Calculating Machine


This manual, full-keyboard non-printing modified stepped drum calculating machine has a metal frame painted black and a steel keyboard painted green. In the six columns of black and white color-coded keys, keys for odd digits are concave and those for even digits are relatively flat. At the bottom of each column is a red clearance key. Metal rods between the rows of keys serve as decimal markers.

Three key stems are in a column to the right of the number keys. One clears the entire keyboard. The other two are set to determine whether or not the keyboard clears after each calculation. The keys for these key stems are missing. A metal lever is to the right of the keyboard and a metal knob to the left. The operating crank on the right side rotates clockwise for addition and counterclockwise for subtraction.

The carriage in back of the keyboard has a row of 12 white numeral dials for recording results and a row of six numeral dials behind these in the revolution register. The revolution register has black digits for addition and red ones for subtraction. There is no carry in the revolution register. Two thin metal rods between the windows of these registers carry decimal markers. The carriage shift lever is at the front of the machine. To the right of the result register is a knob for lifting the carriage. A zeroing crank for dials on the carriage is on its right side. A bell rings when the result passes through zero (as in over-division). The machine has four rubber feet.

A mark on the front reads: MONROE. A mark on the back reads: MONROE (/) Calculating Machine Company (/) New York, U.S.A.. Stamped on the edge of the carriage on the back left is the serial number: 77811.


J. H. McCarthy, The Business Machines and Equipment Digest, 1928, pp. 9-29 to 9-33.

Date Made: 1926

Maker: Monroe Calculating Machine Company

Location: Currently not on view

Place Made: United States: New Jersey, Orange

Subject: Mathematics


See more items in: Medicine and Science: Mathematics, Calculating Machines, Science & Mathematics


Exhibition Location:

Credit Line: Gift of Walton G. Banks, Inc.

Data Source: National Museum of American History

Id Number: 1984.0682.04Catalog Number: 1984.0682.04Maker Number: 77811Accession Number: 1984.0682

Object Name: calculating machine

Physical Description: plastic (overall material)wood (overall material)aluminum (overall material)steel (overall material)Measurements: overall: 19.5 cm x 39 cm x 40 cm; 7 11/16 in x 15 11/32 in x 15 3/4 in


Record Id: nmah_690571

Our collection database is a work in progress. We may update this record based on further research and review. Learn more about our approach to sharing our collection online.

If you would like to know how you can use content on this page, see the Smithsonian's Terms of Use. If you need to request an image for publication or other use, please visit Rights and Reproductions.