Stereographs are double photographs printed side-by-side that create a single three-dimensional image when viewed through a stereoscope. The stereoscope allows the two photos to be viewed close enough to the viewer's eyes that the images appear to overlap and create the illusion of three-dimensional depth. The curved stereograph card mount, which added to the appearance of depth, was developed in the late 1870s by early stereoview producer B. W. Kilburn and the design quickly became the standard. The format developed along with the early days of photography, with hand-held stereoscopes and stereograph collections being manufactured in the second half of the 19th century.

Underwood & Underwood began in Kansas in 1882 and expanded rapidly, becoming the largest producer of stereographs by 1901. They popularized the concept of stereograph box sets (such as the World War I set) and created numerous subject-based "stereographic libraries." The Keystone View Company was started in 1892 in Pennsylvania and experienced similarly rapid expansion as Underwood. Keystone pursued the box set model as well, but distinguished itself as a developer of educational sets. Ultimately, Keystone survived the closure of other stereograph producers, absorbing their images and rights, and becoming the only remaining major stereograph publisher in the world by 1920.

These two major companies were the primary producers of image sets for the Great War. Underwood photographers produced images of the first few years of the war in Europe (primarily England, Belgium, and Germany), with some pre-entry mobilization in the United States. However, by wartime Underwood & Underwood was constricting and their production became increasingly limited. Keystone's photographer was delayed by permissions until the end of the war, and was only able to produce the majority of his images in 1919. Both companies released sets, with Keystone re-releasing sets including both company's images after they absorbed Underwood & Underwood in 1920-1922. The numerous versions of the World War I set from Keystone were by far the most popular of any of the war stereograph sets, with production continuing all the way to 1935. A large number of the early European views acquired from Underwood were never published by Keystone, due to the fact that their market was largely American. As such their sets featured images largely taken after the U.S. entry in the war and/or of battle locations which were known to American audiences.



Source:

William C. Darrah. The World of Stereographs. William C. Darrah : Gettysburg, Penn., 1977.