Stereograms set up flat, still images in such a way that they appear three dimensional to the viewer. In the mid to late 19th century photographers would shoot scenes using a camera equipped with two lenses a few inches apart. This would simulate our binocular vision and, when viewed through a stereoscope, would appear deeper and more lifelike.
Cartoonist Chris Schweizer has an informative overview of composition problems that can pop up when drawing. The Schweizer Guide to Spotting Tangents defines tangents as
…when two or more lines interact in a way that insinuates a relationship between them that the artist did not intend.
Chris follows with a delightfully illustrated list of ambiguities that often appear when composing pictures. They apply as much to photography as they do to drawing and cartooning and can often be overlooked when making an image.