If you were really concerned that you be able to tell which way was up by shape alone, you want no axes of symmetry. I might suggest a quadrilateral which is like the current bill shape, except with one of the short sides slanted, say making a 60 degree angle at the bottom of the bill, and a 120 degree angle at the top. You could still cut the shape out of a proper sized rectangular sheet with no wastage, provided the width was an even multiple of the long side, by matching them in upside-down right-side up pairs which form a long rectangle. That shape wouldn’t have an narrow “tabs” or “points” on it which would become dog-eared and tear off easily.
If you go into any place that prints things - flyers, business cards, those glossy cardstock ads you get from real estate agents, whatever - it becomes really immediately apparent why most things, including money, are made in rectangles. It is not just the fact that paper comes in long rectangles, it’s that literally all printing equipment is best suited to making big rectangular rolls of paper into smaller rectangles.
Ahhhh! (screams!) But the reason why printing equipment is suited to make rectangles is an effect, not a cause! Sorry, but everyone seems to have gone completely mad in this thread with their cause/effect reversals.
This is like saying that God designed humans with five fingers so that our hands would fit gloves so well.
You want a rectangular note, so that when handing it from one person to another, you can “point” it at them.
Both people can grasp the note, then the first can release.
Try the same with a round or square banknote, and it either is so large it gets too floppy, or it is small enough that the hands get a bit intimate with each other.
If square money was the norm we’d all be talking about the disadvantages of rectangles.
Ya’ll are putting the cart before the horse - paper money is rectangular due largely to tradition. That’s it. It’s like trying to assert the QWERTY keyboard is somehow optimized. Nope. It’s not. It’s tradition.
Broomstick, you think that you are offering an explanation but you are not.
Using your logic, everything *is *because everything has always been that way because of tradition.
Or, extending the analogy that **UltraVires **provided above, humans have five fingers *because *humans have always had five fingers.
Rectangular paper money came into existence at some point. How did that happen? It didn’t float down from the sky, everyone started using it and it became a tradition that has continued to today. Some reasonable explanations have been offered in this thread. If you don’t have an explanation, it’s better to say “I don’t know”.
Some of our consumer products are the result of a lot of research into what would appeal to people, or be comfortable, or is otherwise ideal. Or because the dimensions of the human body cause solutions to a problem to converge.
The shape and size of money is not. That’s why shape/size/color/etc. varies all over the place world-wide. Shapes that can tessellate, which make for minimal waste, will be chosen for that purpose but triangles and squares would also work, but the disadvantages of triangles have already been addressed. That leaves rectangles and the subset of rectangles we call squares. The rest is largely a coin toss.