18

18/24, or 750 parts per thousand — three-quarters gold. The commonest quality jewellery standard in Europe. The scale is karat with a K — a proportion of gold in an alloy — and is not the carat with a C used for gemstones, which is a unit of mass equal to 200 milligrams. A one-carat diamond and 24-karat gold have nothing in common but a homophone.

There are 18 groups in the modern periodic table (worldwide, IUPAC numbering). A group is a column, and elements in one share an outer-electron arrangement — which is what makes the table predictive rather than merely tidy. The lanthanides and actinides are pulled out below to keep the width at 18.

18 is the atomic number of argon, the third most abundant gas in Earth's atmosphere (about 0.93%) and an inert noble gas used in incandescent light bulbs and welding to prevent oxidation.

In Hebrew, the letters chet and yod spell chai — 'life' — and their numeric values, 8 and 10, sum to 18. Within Jewish tradition this makes 18 a customary amount for gifts and charitable donations, which are frequently given in multiples of it. This is a gematria association resting on the letter values, not a rule about how 18 is written: as a numeral, 18 is simply yod-chet.

The age of majority in most jurisdictions is 18 (most countries; some differ).

The voting age in most countries is 18 (worldwide; a substantial minority differ). Austria, Brazil, Argentina and several others allow voting at 16, and a handful of states set it at 20 or 21. The catalogue already carries 18 as the age of majority, which is a related but different threshold — several countries separate the two deliberately.

18 is the most common age of legal adulthood (the age of majority) across countries worldwide, though the exact threshold and its legal consequences genuinely vary by jurisdiction — some countries set it at 16, 19, 20 or 21 for some or all purposes.

There are 18 holes on a full-length golf course (worldwide by convention).

A standard round of golf consists of 18 holes — the reason 'the 19th hole' is a well-known golfing idiom for the clubhouse bar, humorously extending an 18-hole round by one more notional hole.

18 is the only positive whole number that equals exactly twice the sum of its own digits: its digits give nine, and nine doubled is eighteen. Its counterpart is 27, the only number equal to three times its digit sum. Both are consequences of writing numbers in base 10 and would not survive a change of base — which is why a fact about digits is never quite a fact about the quantity.

In Jewish communities, it is traditional that gifts and donations are made in multiples of eighteen, because the letters spelling chai, life, have that numeric value. Chet is eight and yod is ten. The practice is ordinary and widespread rather than doctrinal — a donation of $180 is a recognisable amount rather than a coincidence.

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