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Addograf - C. W. Prewett 1923


The Prewett Addograf circa 1923
Long Beach, Cal. - Pat. Pend.
The front side of the rotating disk is numbered from 1 to 20

The rare Prewett Addograf is a handheld adder from the 1920s. Its inventor, Clay Wiley Prewett, was founder of the Prewett Addo-Graf and System Co. of Los Angeles California.

Prewett Addograf - Reverse Side
On this side, the rotating disk is numbered from 21 to 40

An aluminum disk with a scalloped edge rotates within a simple steel case. The front side of the disk is numbered along the edge from 1 to 20, while the back side similarly bears 21 to 40. On both sides, the top of the case is perforated to expose three numbers at once, either odd or even - e.g. 1, 3, 5 or 2, 4, 6 (see photos).

The Addograf's rotating disk meets a hard stop at 20 as shown above.
The adder is then flipped over and counting may continue up to 40.

The inventor, C. W. Prewett, is the author of The Prewett system - How to Figure Fast, an Instantaneous Calculator (c1923), and this adder was an aide in his reckoning system.


Secret of the Prewett System Revealed
Clay W. Prewett Copyright 1939
704 So. Spring St., Los Angeles
(Courtesy of Edward Root)

How To Figure Fast - The Prewett System

"Worth $100 Costs $1" - Revised 1915 Edition

C. W. Prewett - Houston Texas (source Amazon)


Advertisement for C. W. Prewett's How To Figure Fast
Motor West Magazine 17 September 1917
Motor West (


Instructions for Using the Prewett Addograf System

Suite 311 -- 704 So. Spring Street

TRinity 9821

Los Angeles 14, Calif.
(Courtesy of Edward Root)


Plastic Example of the Prewett Addograf
More Speed - Less Effort
(Courtesy of Bob Fusch)

Marketing Literature for the Prewett ADDOGRAF and System
(Courtesy of Edward Root)
Cardboard Example of the Prewett ADDOGRAF
(Courtesy of Edword Root)
A prominent example of this simple adder is in the mathematics collection of the Smithsonian's National Museum of American History - see the article on American Adders by Peggy Aldritch Kidwell, (ETCetera #31 1995). a page of which is shown below. If you are aware of other examples, please send me a note.

Prewett ADDOGRAF Calculator Los Angeles (source ebay)
Prewett ADDOGRAF Calculator Los Angeles (source ebay)

Prewett's Addometer (source ebay cripplecreekauctions)
Prewett's Addometer with a ridged thumbwheel (source ebay)

Prewett's Addometer, made of aluminum, is yet another variation in the family of Prewett handheld counting aides. Note that the models above and below are of slightly different design and construction.

Prewett's Addometer (source Walter Szrek)
Prewett's Addometer with a scalloped thumbwheel (courtesy of Walter Szrek)