Stereo view cards had two pictures mounted for parallel viewing, on 7 x 3.5 inch heavy cards, usually curved slightly with axis along the long length. The pictures were taken with a two-lensed camera, recording the subject from two points of view separated by about 2.5 inches, duplicating what our two eyes see, this is called a Keyston photograph.
What is keystoning (aka "convergence error")?
Consider what an ideal camera is supposed to do. Take an ideal distortion-free camera. Mark a grid of evenly spaced lines on a wall and point the camera at the grid covered wall. Make sure the camera's film plane is parallel to the wall. Take a picture. Look at the picture and it has a grid on it too. It is a "scale drawing" of the grid on the wall. You can lay a ruler on the picture and you will find all the lines are straight. You can use that ruler to measure all the squares in the grid and you'll find they're all the same size.
Now tilt the camera's film plane so it's no longer parallel to the grid-covered wall. Take another picture. This time the lines in the picture are still straight* but the squares are no longer all the same size. Also, the squares have become trapezoids or other sorts of quadrangles. These shapes are called "keystones".
If you take two pictures from two different angles you will get keystoning. The left camera will make a picture with the small ends of the keystones on the right, while the right camera will have make a picture with the small ends of the keystones on the left.
When you go to merge those two pictures in a viewer, you're going to see a "3D" image.