Found this from the following website:
Mounted Prints
were very common before non-curling gelatin silver prints were perfected; the majority of mounted photos in the 19th century were thin albumen prints which needed a stout backing to prevent deformation. Types of early mounted prints included cabinet cards (4 1/2" x 6 1/2") which were mounted on heavy stock, and smaller, thinner cartes-de-visite (2 1/2" x 4"). Mounted prints in other sizes fall into a number of categories, including Imperial cards (9 7/8" x 7 7/8"); panel cards (7 1/2" x 13", 4" x 8 1/4" or 1 3/4" x 5 1/4"); and boudoir cards (5 1/2" x 8 1/2"). Studio portraits from the early 20th century are usually mounted gelatin silver prints; the cardstock backings on which these photographs were mounted were produced in a great variety of embossed designs. Most studio-made mounted photographs bear the imprint or stamp of the studios which produced them.
http://45semiotic.com/wia/glossary.html
I guess 8x10" actually do qualify as Imperial.