"So we can have a see-through ruler that looks like the line segment but is longer (and is also "one dimensional"). It's for example a very thin glass rod with regular markings (the color of the markings differs from the color of the segment). We place the ruler on top of the line segment and look through it to see how long the line segment is. We never have to use more than one dimension." How would this "see-through ruler" have been made in the first place? (I do think of everything).