6

There are 6 dots in a braille cell (worldwide, standard braille). Two columns of three, giving 64 combinations including the blank. Louis Braille adapted it from a twelve-dot military code, and the reduction to six was the key change: a fingertip can read a six-dot cell without moving.

There are 6 times ten to the twenty-third, the Avogadro constant fixing the mole (worldwide, SI since 2019). Fixed at exactly 6.02214076 x 10^23 when the SI was redefined around constants.

There are 6 legs on an adult insect (insects; a defining characteristic of the class).

There are 6 sides on each cell of a honeycomb (honeybees). The hexagon encloses the most area for the least wall, which was conjectured for two thousand years and proved only in 1999.

There are 6 flavours of quark (the Standard Model). Up, down, charm, strange, top and bottom. Only up and down make ordinary matter; the other four exist briefly in high-energy collisions and in the early universe.

The mass of the Earth is about 6 × 10²⁴ kilograms (the Solar System). First estimated by measuring the gravitational pull of a mountain, and later in a laboratory with lead spheres.

The surface temperature of the Sun is nearly 6 thousand kelvin (the Solar System). The core is nearer 15 million kelvin, so the surface is the coolest part of the star that is visible.

There are over 6 thousand exoplanets confirmed (worldwide, NASA Exoplanet Archive). Figure as of 2024. Almost none was known before 1995. The count is a fact about detection methods as much as about planets: transit and radial-velocity surveys find large, close-orbiting planets most easily, so the catalogue is skewed toward them.

Honeycomb cells are six-sided, and the reason is geometric rather than behavioural: of the three regular polygons that tile a plane without gaps, the hexagon encloses a given area with the least total wall. That efficiency claim was long conjectured and was proved by Thomas Hales in 1999.

6 is the atomic number of carbon, the element that forms the structural backbone of every known living organism and the basis of organic chemistry.

A web colour is written as six hexadecimal digits — #RRGGBB — two for each of red, green and blue. Six is the count because each channel needs a byte, and one byte takes exactly two hex digits, giving 16⁶ ≈ 16.8 million expressible colours.

In the Genesis creation narrative (Hebrew Bible/Old Testament), the world is described as created over six days, with the seventh set aside for rest.

There are 6 sides on a snowflake (water ice, worldwide). Six-fold symmetry follows from the hexagonal crystal lattice of ordinary ice, which is why it is universal rather than common. The endless variety is in the branching, not the count.

The radius of the Earth is about 6 thousand kilometres. 6,371 kilometres as a mean value. Eratosthenes got within a few per cent of it around 240 BC using shadows and a measured walking distance.

There are 6 balls in a cricket over (worldwide, ICC; Australia used eight-ball overs until 1979). It has been four, five, six and eight at different times and places. Six became the international standard only in 1979, which is recent for a game whose laws date from 1744.

There are 6 strings on a standard guitar (worldwide, the standard classical and steel-string guitar). Five-course guitars were standard into the eighteenth century. The six-string tuning E-A-D-G-B-E is mostly fourths with one third, an asymmetry that makes chord shapes awkward and scale shapes convenient.

6 is the smallest perfect number — equal to the sum of its proper divisors, 1 + 2 + 3. Euclid's theorem in the Elements shows that whenever 2ᵖ − 1 is prime, 2ᵖ⁻¹(2ᵖ − 1) is perfect, and 6 is that construction's first case. Perfect numbers remain rare: only 51 are known, every one of them even, and whether an odd perfect number exists is still open.