From collection crpa-formats
Card photographs, generally portraits, which measure 4 1/4 by 6 1/2 inches, including the mount. They were introduced by F. R. Window in 1867 as larger alternatives to the "cartes-de-visite," which measured 3 1/4 x 2 1/4 inches. The larger size was considered more appropriate for display, allowed for group portraits, and permitted the image to be retouched. The size remained popular until World War I.
