system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why paper and photographs are predominantly rectangular rather than square shaped?
gpt
Photographs tend to be primarily portraits or landscapes. The human face on average is longer than it is wide, and therefore a vertical rectangular format works best for depicting it (this is true in painting, drawing and relief sculpture as much as in photography). Landscape photography works best in a horizontal format, since what tends to be most interesting (middle distance and horizon) is horizontal, and you can crop out immediate foreground and all of that uninteresting sky.  

Paper tends to be rectangular because of writing and printing. In theory, a square shape would work just as well as a rectangular (usually vertical) shape for printing books and manuscripts -- and in fact there are quite a few square-format books that have been printed. But various conventions of writing and publishing in most languages have made it so the layout of text tends to be vertical: page numbers, footnotes, comfortable line length for reading in languages that print horizontally, etc.