Just like the camera and other elements in the design https://news.ycombinator.com/item?id=5883125 the shape of the notes is a pastiche of an (I think) roughly mid-'60s commercial-art style.
Just like the camera and other elements in the design https://news.ycombinator.com/item?id=5883125 the shape of the notes is a pastiche of an (I think) roughly mid-'60s commercial-art style.