| Name and Model Number | Keuffel & Esser Cox Duplex |
| Country of Origin | USA |
| Date of Manufacture | 1891 |
| Front Scales | A [ B C ] D |
| Back Scales | A [ BI CI ] D |
| Scale Length | 25 cm |
| Type of Rule | Slide Rule |
| Construction | Celluloid laminate on mahogany |
| Area of Use | Generic |
| Comment | Patented by William Cox October 6, 1891. Robert Otnes relates that Hans Dennert of Dennert & Pape in Germany told him:
(1)The basic parts of the rule (the three bars with the scales, without letters and numbers) were made in Germany at D&P and shipped unassembled to K&E in New York in order to avoid US duty (tax). (2)These were then finished by K&E employees: the letters and numbers were stamped on, and the brass cursor and the two end plates were added. (3)There is nothing about D&P or Germany on the rule. Note: By the time this rule was made, D&P had slide rule dividing engines, probably based on those by Lenoir circa 1820. Later rules with partial German content were so marked, but that is another story. Only two of these slide rules are known. Both appear in this Rarities Gallery. |
| Rarity | RRRR |
| Donor | Collection of Robert Otnes |
| Match Number | 388 |
| Thumbnail (Click Picture for Details) |  |
| In conjunction with our 'donors' we assign a rarity rating to each item, based on the system devised by Herman van Herwijnen |
| (1929 - 2004), renowned Dutch collector, author, and creator of Herman's Archive, a collection of 5,000 slide rule images on CD. |
| • RR = Rare - 1 of 25 collectors has one; it may take a year to find an example in the marketplace. |
| • RRR = Very Rare - only a few known to collectors; may be several years before it comes up for sale. |
| • RRRR = Extremely Rare - a museum piece; unlikely that an example will come up for sale. |