International Paper Sizes (e.g. A4) use a 1:√2 ratio. If you cut them in half crosswise, the same ratio will be maintained.
It’s great for scaling up or down.
21st Prime Number
73 is the 21st prime number. Its mirror, 37, is the 12th and its mirror, 21, is the product of multiplying 7 and 3 and in binary 73 is a palindrome, 1001001, which backward is 1001001. 73 is also the best number. Itself and its mirror and 100 plus both numbers are all primes (73, 37, 137, 173).
If you take 73 and 100 more than its mirror, 37 (137) and multiply them together you get 10,001. Which leads to a calculator trick I learned when I was young: Take any four-digit number and multiply it by 73, then multiply the result by 137, your result is the four-digit number repeated twice.