Monroe Model D Calculating Machine

This is an early model of the full-keyboard, modified stepped drum, manually operated, non-printing calculating machine manufactured by the Monroe Calculating Machine company on patents of Frank S. Baldwin.
The machine has a steel frame, painted black. The eight columns of plastic keys are colored black or white according to the place value of the digits represented. The key stems are banked, standing up further toward the back of the machine. At the front of the keyboard is a row of red keys, each of which clears the column above. On the right side, in this row, is a red key for clearing the entire keyboard.
Behind each column of keys is a double disc with teeth of varying length protruding from it that serves as a stepped drum. One disc has five teeth on it, and the other has four. Pushing the appropriate key causes one or both of the discs to approach one another so that they are engaged when a crank on the right side turns, entering the appropriate number into the mechanism.
Between the rows of keys are metal strips, painted black on one side and white on the other. They rotate to serve as decimal markers. Two silver-colored knobs have arrows on them. The one on the right is used for automatic keyboard release in continuous addition. The one on the left is the register key for the item counter.
In back of the keyboard is a carriage with 16 black total register dials and eight white and red revolution register dials behind the total dials. Between the windows for these registers are two thin metal rods that carry decimal markers. Another handle on the right side of the carriage zeros the total register or the revolution register, depending on the direction in which it is turned. A knob on the right side of the carriage is used to lift it. A crank at the front of the machine rotates to move the carriage.
A mark on the front of the machine reads: MONROE (/) REGISTERED TRADEMARK (/) Calculating Machine Company (/) New York, U.S.A. A sticker toward the left on the carriage reads in ink: CLEARING. A mark at the center of the back reads: D1749. A mark under the carriage on the right reads: 1749. A sticker on the back of the machine reads: PATENTED JUNE 16, 1908 (/) OTHER U.S. AND FOREIGN PATENTS (/) PENDING.
Compare MA*307385. The latter machine has a different carriage shift mechanism, a lower serial number, and a different color-coding of the keys. It has no decimal dividing rods between the keys, and no mention of a trademark. It has a different way of marking decimal divisions on the carriage and has been assigned a slightly earlier date.
Monroe first used the name Monroe to describe its products in 1913. The company applied for a trademark on April 7, 1919, and was granted it onJanuray 20, 1920.
E. Martin, The Calculating Machines (Die Rechenmaschinen), trans. P. A. Kidwell and M. R. Williams, Cambridge: MIT Press, 1992, p. 265.
U.S. Patent and Trademark Office, Trademark Registration 129029, Serial #71117235.
Currently not on view
date made
Monroe Calculating Machine Company
place made
United States: New York, New York
Physical Description
wood (overall material)
steel (overall material)
aluminum (overall material)
plastic (overall material)
overall: 16.5 cm x 43 cm x 32.3 cm; 6 1/2 in x 16 15/16 in x 12 23/32 in
ID Number
catalog number
maker number
accession number
See more items in
Medicine and Science: Mathematics
Science & Mathematics
Calculating Machines
Data Source
National Museum of American History, Kenneth E. Behring Center


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