Did you know that 190 out of 200 nations in the world have either red or blue on their flags? (The wheel in the middle of India’s flag is blue, for example, and the Vatican flag has a red cord hanging from the keys.) Did you know that four of those 190 — Andorra, Chad, Moldova and Romania — have pretty much the same blue-yellow-red tricolour? Or that the stripes of the French flag are not of even width, but are proportioned 30-34-37? It’s an optical illusion: if the red, white and blue are of equal breadth, the flag looks...