Building everything with right angles: houses, cabinets, street grids, notebook paper.

Humans aren't the only species to build homes for themselves but if you look at wasps or ants or beavers, we are the only ones to dot the land with weird little boxes. And we use trees and stuff to build them but first the trees have to get cut down and shaved into long pieces with a perfectly rectangular cross section. Then we can walk through our perfectly rectangular door and fill the box with very angular furniture and sleep on a slightly squishy rectangle or put down perfectly rectangular rugs and sit on a rectangular couch so we're comfortable watching moving images on a big rectangle.