Photos With Square Mounts
Somehow, about the year 1890, photographers got the strange idea that images might look good mounted on square card-mounts. Card mounts came in various sizes, but they had always been rectangular. Esthetically it was a lousy idea, but for a while in the 1890s and early 1900s there was a fad for square card mounts.

Here is a typical example from 1890. For this one, the print itself is also square, though the subject was arranged diagonally, which is typical of the earliest square mounted photographs. Others had round images — a take-off on the new Kodak no doubt, though smaller. These cards are typically between 2.5 inches and 3.5 inches square. One also sees amateur square mounts in various sizes, both larger and smaller, but this is typical of the mass-produced market (though amateurs might purchase these mounts for their own use as well as professional photographers, the other sizes referred to are hand-cut).
These became very popular in the early 1890s, then fell-off in mid-decade, as slightly rectangular versions were produced (e.g. 2.5 by 3 inches, or 2.75 by 3.25). By late 1890s the square mounts make a re-appearance, but now they typically have rectangular images mounted on them, and many are larger sizes, the most popular being 5 by 5 inches.
Most of the square card mounts from the early 1890s were white, or buff-colored as in the example above. By the late 1890s white was still most popular, but brown and gray shades are occasionally seen. After 1900 they were almost always brown or gray.

The above example is from 1906, and one of the latest I have seen for this style. The image itself is also square, which was not very typical after 1900, which makes me suspect the date may be incorrect. The women’s dresses look 1890s, but that is not uncommon for someone of that age. The image is too faint to be able to say for sure.
In any case, the vast majority of square mounted images date from 1890 to 1905. They most often have white pebbly-surfaced card faces. A large percentage seem to have been printed by amateurs, and have a snapshot sensibility, as opposed to formal studio portraits.
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