Imperial Dual Unit typewriter, 1955

In the mid to late 1950s, the Imperial Typewriter Company introduced the Dual Unit Typewriter, a rare twin keyboard typewriter which could be used in academic and scientific fields. It was initially requested, contemporary articles claimed, by the UK Atomic Energy Authority.

The dual-unit typewriter was a specialised piece of office equipment and cost about two to three times the cost of an ordinary typewriter. As such, it was mostly limited to offices with a large amount of typing of technical and/or mathematical papers, where the cost could be justified by the saving in time and effort. Customers included universities, technical companies, and government departments around the world. This example was used in the Blackett Laboratory at Imperial College, London and has the standard 92-character typewriter keyboard alongside a keyboard with special symbols required for scientific papers.

The dual-unit typewriter consisted of two physically separate, different, and interchangable keyboards mounted side-by-side: one universal standard QWERTY keyboard on the left and one additional keyboard on the right which could have mathematical, scientific or phonetic symbols. In operation, the carriage was moved from one keyboard to another as required, without any need to adjust the register.

The individual keyboard units were interchangable and were designed to be removable and lock into position. They could only type when the transferrable carriage was transferred from one keyboard to the other and was locked into place over the keyboard to be used. Once locked into place the keyboards were freed to type. At the same time, the carriage itself unlocked and moved in a normal way. This locking design ensured that the precise position of the paper was transferred from one keyboard to the other, while at the same time ensuring accidental typing could not take place.

Claude Wellington Robert Brumhill first patented this design in England in 1949, toward the end of his eight-year stint as lead design engineer for Imperial Typewriters in Leicester. It was the pinnacle of his achievements for Imperial. He had applied for a US patent for the double (and triple) keyboard in December 1950 and the patent was issued in January 1954. The Imperial Dual Unit Typewriter seems to have been in limited use in the mid to late 1950s.