Since first seeing the Ulam spiral on Numberphile, it wasn't surprising to me that lines like that would appear. Being that prime numbers are all the combinatio

Since first seeing the Ulam spiral on Numberphile, it wasn't surprising to me that lines like that would appear. Being that prime numbers are all the combinations by which you cannot stack a two dimensional area of pixels in a fully filled rectangle. e.g. ... ... ... 9 is not prime, because you can fill all rows evenly. ... .. .. 7 is, because you can't fill all the rows evenly. So the gaps make lines... Or to put it another way, the lines in the Ulam spiral are the pixels next to the corner pixels that make full rectangles that can't be made full rectangles another way.