12

There are 12 eggs in a standard box (United Kingdom and United States retail). The dozen survives in eggs long after most goods moved to metric quantities, because the carton is moulded for twelve and moulds outlast conventions.

There are 12 items in a dozen.

The year carries twelve months because a solar year holds roughly 12.37 lunar cycles, and twelve is the nearest whole number to that figure. The leftover fraction is why purely lunar calendars drift against the seasons and why lunisolar calendars insert an occasional thirteenth month.

Twelve divides evenly by two, three, four and six, which is why it persisted for eggs, inches, hours and months long after decimal counting was established. Dozen-based measure retreated but did not disappear.

Western equal temperament divides the octave into twelve equal semitones — the twelve keys, white and black, before the pattern repeats. Twelve is a compromise: it is the smallest division that gets acceptably close to the simple frequency ratios the ear finds consonant. Other traditions divide the octave differently.

It is often claimed in popular writing about ancient civilisations that the base-twelve counting of some ancient systems is evidence of a lost advanced civilisation. Duodecimal counting has a mundane and well-evidenced explanation: twelve divides by two, three, four and six, and finger-joint counting on one hand gives twelve naturally. Both are documented; neither requires anything lost. This is a modern popular interpretation, not a historical or doctrinal claim.

In English-speaking commerce, it is traditional that goods are sold by the dozen, and thirteen is given as a baker's dozen. The baker's dozen is recorded from the thirteenth century. The extra loaf is usually explained as insurance against penalties for selling underweight bread. Twelve itself persists because it divides by two, three, four and six, which matters when goods are split by hand rather than weighed. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

In Chinese-, Japanese- and Korean-speaking cultures, it is traditional that years are named in a repeating cycle of twelve animals. The animal is a year's earthly branch. Pairing the twelve with the ten heavenly stems gives the sixty-year cycle, so a birth year is fully specified only by both. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

Base-12 (duodecimal) counting has been used historically alongside base-10 because 12 divides evenly by 2, 3, 4 and 6, making common fractions easier to express — a likely reason for 12-based units such as the dozen, the 12-hour clock face, and 12 inches in a foot.

Typically, there are 12 pairs of ribs in a typical human ribcage (humans).

There are 12 people who have walked on the Moon (1969 to 1972, all Apollo missions).

The mass number of the carbon isotope that anchored the atomic mass unit is 12 (worldwide, 1961 to 2019). Carbon-12 was defined as exactly 12 units, so every other atomic mass was measured against it. The 2019 SI revision moved the anchor to a fixed Avogadro constant, and carbon-12 is now measured rather than definitional.

The diameter of the Earth is about 12 thousand kilometres. 12,742 kilometres through the middle. The Earth is very slightly wider across the equator than pole to pole, by about 43 kilometres.

12 is the atomic number of magnesium, an essential nutrient and a common lightweight structural metal.

There are 12 inches in a foot (imperial and US customary units). Twelve divides by two, three, four and six, which mattered when measurements were halved and thirded by eye rather than calculated.

There are 12 hues on a standard artist's colour wheel (worldwide, art and design convention). Three primaries, three secondaries and six tertiaries. It is a convention of pigment mixing rather than of light: the additive primaries a screen uses are red, green and blue, which mix to white rather than to mud.

In the Hebrew Bible, the descendants of Jacob are counted as twelve tribes, though the lists in different books do not name quite the same twelve. Levi is included in some lists and omitted in others, and Joseph is sometimes replaced by his two sons — so the number is stable while its membership is not. This is a scholarly account rather than the tradition's own framing.

In the New Testament, Jesus is described as choosing twelve apostles, a count generally read as deliberately echoing the twelve tribes. This is a scholarly account rather than the tradition's own framing.

In the Hebrew Bible, the people of Israel are described as twelve tribes descended from the sons of Jacob. The lists in the text do not all agree. Levi is sometimes omitted as the priestly tribe and Joseph is sometimes split into Ephraim and Manasseh, which keeps the total at twelve by different routes. This is a scholarly account rather than the tradition's own framing.

In the New Testament, Jesus is described as choosing twelve apostles. The choice of twelve is generally read as deliberately echoing the twelve tribes. The named lists in the gospels differ slightly from one another, and the replacement of Judas restores the count rather than leaving eleven. This is a scholarly account rather than the tradition's own framing.

Twelve recurs as a structuring count in Jewish and Christian scripture: the twelve tribes of Israel descended from Jacob's sons, and the twelve apostles of the Gospels. Later Christian writing reads the second as deliberately echoing the first.

In ancient Greek religion, twelve gods were counted as the principal Olympians, though which twelve varied by city and by period. Hestia and Dionysus are the usual swap. A civic list rather than a fixed pantheon, which is why no two ancient sources agree completely. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

In Buddhism, dependent origination is set out as a chain of twelve links. The chain is usually depicted running round the rim of the wheel of life. Read forwards it accounts for the arising of suffering; read backwards it is the account of its cessation. This is a scholarly account rather than the tradition's own framing.

Presented as belief or tradition, not as an established fact.

There are 12 steps on the Beaufort wind force scale (worldwide, meteorology). Force 0 is calm and force 12 is hurricane. Beaufort defined it in 1805 by what the wind did to a ship's sails rather than by a speed, which is why it remained usable long before anemometers were common.

The depth of the deepest borehole ever drilled is about 12 kilometres (Kola Peninsula, Russia). Abandoned in 1992 when the rock at the bottom proved hotter and more plastic than predicted. It is roughly a five-hundredth of the way to the centre of the Earth.

A jury of twelve is the common-law standard, used for Crown Court trials in England and Wales and for federal criminal trials in the United States. The number is inherited convention rather than statute-derived reasoning, and it is not universal: Scotland seats fifteen, and several US states allow six in civil cases.

Twelve-member juries are standard across common-law jurisdictions and the number has no statutory derivation — it was inherited from medieval English practice and has resisted reform attempts in several countries. Scotland uses fifteen, which shows the figure is convention rather than necessity.