The 20

Calcium, element 20 — bone, heartbeat, and a doubly-magic nucleus ChemistryLife

Calcium sits at atomic number 20, and it is the most abundant metal in your body — about a kilogram of it, nearly all locked into the mineral of bone and teeth. But the fraction dissolved in your cells and blood does the most dramatic work: a pulse of calcium ions is what makes a muscle contract, so every heartbeat is element 20 flooding in and being pumped back out. Calcium also triggers the release of neurotransmitters and the clotting of blood — a signalling switch the body cannot do without.

Its most common form, calcium-40, is a physicist's marvel: with 20 protons and 20 neutrons it is doubly magic (see below), which is why it is so stable and abundant — the calcium in your bones was forged in stars and has essentially never decayed since.

The twenty amino acids — the alphabet of all life Life

Every protein in every living thing — the enzymes, the muscle, the antibodies, the machinery of the cell — is a chain spelled out from an alphabet of exactly 20 amino acids. The genetic code in your DNA is, at bottom, a lookup table that translates three-letter words of genetic code into these 20 building blocks, strung in order to fold into a working molecule. From the same 20 letters, life writes a near-infinite library.

Why 20, and not ten or fifty, is one of biology's deep open questions — the number seems to be a frozen accident locked in near the origin of life. (A handful of organisms sneak in a 21st or 22nd, but the universal set is 20.) It is one of the most fundamental counts in all of nature: the whole of you, and every creature you have met, written in an alphabet of 20.

Twenty faces, twenty corners — the icosahedron and its twin Mathematics

20 is a number of the Platonic solids. The icosahedron has 20 identical triangular faces — the shape of the twenty-sided die that has decided the fate of adventurers in tabletop games for half a century. Its dual, the dodecahedron, turns faces into corners and so has 20 vertices. These are two of the only five perfectly regular solids that can exist in three dimensions, and 20 sits at the heart of the most intricate pair.

That icosahedral geometry is not just a game piece: many viruses build their protein shells as icosahedra, because 20 triangles are the most efficient way to wrap a symmetric container around a genome. The common cold and the herpes virus are, in shape, tiny twenty-sided dice.

A magic number — why calcium-40 is so stable Physics

Just as electrons are especially stable in the closed shells that make the noble gases, protons and neutrons fill shells inside the nucleus — and the counts that exactly close a shell are called magic numbers: 2, 8, 20, 28, 50, 82, 126. A nucleus with a magic number of protons or neutrons is unusually tightly bound; one that is magic in both is “doubly magic” and exceptionally stable.

Twenty is one of these numbers, which is why calcium-40 — 20 protons and 20 neutrons — is doubly magic and the overwhelmingly common form of the element. The magic numbers were a triumph of the nuclear shell model in the 1940s, and 20 was one of the clues that first revealed the nucleus, too, is built in shells.

Counting by scores — the vigesimal world Culture

Long before ten won, many peoples counted in twenties — a score — because a person has 20 fingers and toes. The Maya built their whole mathematics and calendar on base 20, stacking twenties to reach into the millions. The count survives, half-buried, in living languages: the French say quatre-vingts — “four twenties” — for 80, and English kept “score” long enough for Lincoln to open the Gettysburg Address with “four score and seven years ago.”

The vigesimal habit turns up all over the world — in Welsh, Basque, Danish, and Georgian counting — a quiet fossil of the oldest counting tool there is: the whole set of fingers and toes, 20.

Twenty-twenty — the yardstick of sight Life

“20/20 vision” is the benchmark of normal eyesight: it means you can read at 20 feet the line on the chart a typical eye reads at 20 feet. Better-than-normal sight is 20/15 (you see at 20 feet what others need 15 to see); worse is 20/40. The 20 is simply the fixed testing distance of the Snellen chart, and the number has become shorthand, far beyond the optician's, for seeing clearly — as in “hindsight is 20/20.”