Fun Facts About the Number 21: Blackjack, Dice, Salutes and a Half Marathon
Twenty-one shows up in more places than any number its size has a right to: cards, dice, birthdays, salutes, a half marathon, a famous experiment that was wrong. We named a daily puzzle after it, so we collected the best of them.
The spots on a die add up to 21
1 + 2 + 3 + 4 + 5 + 6 = 21. That also makes 21 a triangular number — the sixth one — because you can stack it as rows of 1, 2, 3, 4, 5 and 6 dots into a triangle. The next triangular number is 28; the one before is 15.
Blackjack is really called twenty-one
The game is "twenty-one" almost everywhere — vingt-et-un in French, veintiuna in Spanish. Cervantes mentions card sharps playing veintiuna in a story published in 1613, which makes it at least four centuries old. "Blackjack" was an American bonus payout for a hand with the ace of spades and a black jack, and the name stuck to the whole game.
XXI in Roman numerals
X is ten, I is one, and you add them: XXI = 10 + 10 + 1. Roman numerals mostly work by addition like this — the subtraction cases (IV, IX, XL) are the exception. More on that here.
The 21-gun salute
The highest military honour in most countries. The tradition comes from naval custom: ships fired their guns to show they were empty and therefore harmless, and the numbers settled over centuries into odd counts, with 21 at the top. It's fired for heads of state and, at funerals, for a very short list of people.
Twenty-one grams — the weight of a soul, supposedly
In 1907 a Massachusetts doctor named Duncan MacDougall weighed six dying patients on a bed-scale and reported that one of them lost about three-quarters of an ounce — 21 grams — at the moment of death. His sample was tiny, his measurements were inconsistent, and nobody has reproduced it. The number lived on anyway, including as the title of a 2003 film.
Legal drinking age in the United States
The National Minimum Drinking Age Act of 1984 pushed every state to 21 by tying it to federal highway money. Most of the rest of the world sits at 18. It is one reason "21st birthday" carries weight in America that it doesn't elsewhere — though "key of the door at 21" was an older British idea, from when 21 was the age of legal majority there, until 1970.
A half marathon is 21.1 km
21.0975 kilometres, to be exact: half of the 42.195 km marathon. That odd marathon distance dates from the 1908 London Olympics, when the course was arranged to start at Windsor Castle and finish in front of the royal box.
It's a Fibonacci number
1, 1, 2, 3, 5, 8, 13, 21, 34. Each number is the sum of the two before it. Twenty-one is the eighth term, and it's also 3 × 7 — the product of the first two odd primes — which is a nice coincidence rather than a deep fact.
Table tennis and badminton games went to 21
Badminton still does: games are to 21 points, best of three. Table tennis switched from 21 to 11 in 2001 to make matches shorter for television, which is why anyone who learned the game before then plays to the wrong number.
In binary it's 10101
16 + 4 + 1. A palindrome, which almost nothing is in binary, and the first number after 5 (101) to alternate ones and zeros all the way along.
The smallest perfect squared square uses 21 squares
A "perfect squared square" is a square tiled by smaller squares that are all different sizes. The smallest one possible is 112 × 112 and uses exactly 21 squares; it was found by A. J. W. Duijvestijn in 1978 after decades of searching, with a computer.
Twenty-one letters
The Italian alphabet has 21 letters — it skips J, K, W, X and Y, which only appear in borrowed words. The Hebrew alphabet has 22, Latin had 21 for a long stretch before Y and Z were added for Greek loanwords, and the genetic code has 20 standard amino acids plus a 21st, selenocysteine, that's encoded in a roundabout way.
Pop culture collects it
Adele's 21 (2011) was the best-selling album of the decade in several countries. 21 Jump Street launched Johnny Depp and then became a comedy franchise. 21 (2008) is the film about the MIT blackjack team. Twenty One Pilots, 21 Savage, and the 21st century itself round it out.
And one grid a day
Our contribution: a 4×4 grid of digits where you draw a path that adds to exactly 21, scored by how many tiles you used. Three tiles is easy. Eight is the flex. Today's is here.
Questions
Why is 21 the target in blackjack?
Nobody recorded the reason. The game descends from older European "reach a total without going over" card games — 31 was also common — and 21 is the version that spread through French casinos and then America. It is simply the number that won.
Is 21 a prime number?
No. 21 = 3 × 7. It is a Fibonacci number, a triangular number (1+2+3+4+5+6), and the sum of the spots on a six-sided die.
Is the 21 grams soul experiment real?
The experiment happened — Duncan MacDougall published it in 1907 — but its results are not accepted. Six patients, inconsistent readings, and no one has reproduced the finding. The 21 grams figure is folklore with a footnote.