About Stereoviews
Stereoview cards featured two photographs printed side by side, one for the left eye, and one for the right. When looking through a stereoscope, the two flat pictures blend into a single image giving the illusion of depth.
This photographic technique originated in the 1840s and was popular until the 1930s, when movies and other modern media took center stage. In the late 19th century, looking at stereoviews of famous and faraway landmarks was a popular pastime.