is the 3rd star number (3 x 3 + 4 x 3).
is a Fibonacci number, and the smallest number of differently sized squares that are needed to tile a square.
is the number of spots on a standard cubical die (1+2+3+4+5+6).
is the number of ways of choosing a set of 5 elements from a set of 7 elements.
is x for: y=2x-22 and 3x-4y+17=0
= 112 - 102 = 52 - 22
= 40 + 41 + 42
= 62 - 52 + 42 - 32 + 22 - 12
If a square number ends with xyxyxyxyxy, then xy = 21, 29, 61, 69 or 84. For instance: 508,853,9892 = 258932382121212121
221 - 21 is prime.
The smallest simple perfect squared square (discovered by A. J. W. Duijvestijn in 1976 - Journal of Combination Theory) is tiled using 21 different squares.
"When I Was One-and-Twenty" is a dramatical verse by Alfred Edward Housman about the futility of love, youth, experience, and the irony in living life.
'Twenty-one' is the name of a card game (also called 'blackjack').
the German 'Twenty-One Club' (Zwanzigeins e. V.) is dedicated to the promotion of a new way to spell numbers in order to make mental arithmetic easier. The supporters of the club want Germans to say zwanzigeins (twenty-one) instead of the current einundzwanzig (one-and-twenty).