Notes: "The Cox Duplex was K&E’s first duplex slide rule, and it was a true duplex, meaning that calculations started on one side could be continued on the other side through use of the cursor. It was designed by William Cox, a contractor to K&E, and patented in 1891. The Duplex first appears in the 1895 edition of the K&E catalog. The purpose of the Duplex was to make solving certain types of problems (such as A x B x C =X) possible with a single setting of the cursor. The Cox Duplex was initially made for K&E by Dennert & Pape in Germany. Sometime around 1900 K&E took over making the Duplex themselves. "
