This will be the major topic of this site, and will be based upon quite some information from the Zeiss Historica Society and the ZICG, an active Yahoo group.
The Zeiss-Ikon Aktiengesellschaft was founded in 1926 at Dresden when four of the leading German camera makers, Contessa-Nettel A.G., Stuttgart, Ernemann Werke A.G., Dresden, Optische-Anstalt C.P. Goertz, Berlin and the Ica Aktiengesellschaft, Dresden, united under the banner of Zeiss, whose Tessar and other lenses had long equipped the world's best cameras. Lens manufacture continued at the Carl Zeiss, Jena works, with research laboratory at Saalfeld, but the holding company became Zeiss-Ikon, a reasonable choice of name since Ikon suggests the Greek for image or picture.
When the Goerz, Ernemann, Contessa-Nettel and Ica companies came together in 1926 to form Zeiss-Ikon the joint catalogue included more than 120 models, some of which had been household names for 20 years. Some of these were dropped, others lived on into the thirties. All are collectors' pieces today.
Rationalisation was of course overdue. In the mid-1920s photography, like motoring, was changing. No longer a pastime for enthusiasts, it became a mass movement as roll film gradually superseded plates. The change was gradual. Amateurs often preferred plates because they offered a wider choice of emulsions and speeds, and plates always remained flat. They could be focused precisely on a ground-glass screen, and a flat-folding plare camera was slimmer than its roll film equivalent. There was also a certain magic about the ritual of tripod, focusing cloth and dark slide which distinguished the serious worker. Glass plates were heavy, of course, and required a darkroom for loading; but plate cameras would also take 'film pack', factory-made packs of cut film interspersed with black paper which slid into the camera in the same way as a dark slide. By pulling a paper tab a film was brought into position for exposure and a further pull carried it to the back of the pack. The front of the adapter carried a draw slide like that of an ordinary plate carrier, and negatives could be developed singly instead of waiting for the end of a roll. Nearly all portable plate cameras could be used in this way, and many roll film cameras, too, could be fitted with film pack adapters. 'Roll-holders' too were sold so that plate cameras could be used with roll film. Cameras led a versatile life in the twenties. And miniature cameras for cine film were already in the shops.
The above is from the book of D.B. Tubbs, Zeiss Ikon cameras 1926-39 published by Hove camera foto books.