八 is one of the rare numerals whose original meaning was not a count but a concept: dividing. The oracle-bone graph shows two strokes turning away from each other, a picture of something split apart and separating into two. The Shuowen Jiezi defines it precisely this way: 八，别也, "eight is division," two halves drawing back from one another. The numeral eight was written with this dividing-graph as a phonetic loan, and the borrowing was so complete that the original sense had to be respelled.

That original dividing sense survives in the cognate 分 fēn (to divide), which is built from 八 (the dividing strokes) over 刀 (a knife): to cut apart. So when 八 became locked to the number, 分 took over the act of dividing, with the knife added to make the cut explicit. 八 also serves as a productive component meaning "to separate" inside other characters.

As a radical, 八 sits at the top or bottom of many characters (六, 公, 兵, 共), often with no connection to either eight or dividing, simply as a shape. But its own history is clear: two strokes parting, a graph for separation borrowed to write the number that, by an accident of sound, would become the luckiest in the language.

The most important set of eight in Chinese thought is 八卦 bāguà, the eight trigrams. Each trigram is a stack of three lines, every line either solid (yang) or broken (yin). With two choices over three positions, there are exactly eight combinations, and these eight exhaust the basic configurations of yin and yang. They are conventionally named 乾 (Heaven), 坤 (Earth), 震 (Thunder), 巽 (Wind), 坎 (Water), 离 (Fire), 艮 (Mountain), and 兑 (Lake).

The eight trigrams are the building blocks of the Yijing: paired two at a time, they generate the sixty-four hexagrams that make up the classic. They were also arranged into spatial diagrams, the Earlier Heaven and Later Heaven sequences, that map the trigrams onto directions, seasons, and family roles. These diagrams underlie feng shui, the bagua mirror hung over doorways for protection, and the eight-sided arrangements found in temple architecture and martial arts.

In everyday modern Mandarin, 八卦 has taken a startling second life: it now means gossip. To 八卦 someone, or to be a 八卦 person, is to trade in idle talk and others' private affairs. The path from the cosmic trigrams to celebrity gossip runs through Cantonese usage and the sense of the bagua diagram's wandering, all-encompassing reach, but for a learner the result is one word with two utterly different registers.

Eight is the most coveted number in modern Chinese culture, and the reason, as with four and six, is pure homophony. 八 bā sounds close to 发 fā, the first syllable of 发财 fācái (to get rich, to make a fortune) and 发达 fādá (to prosper). In Cantonese the resemblance is even stronger. To possess eights is, by association, to possess prosperity.

The desire is intense and economically real. Phone numbers, license plates, and addresses heavy with eights sell for large premiums; sequences like 8888 fetch sums that can dwarf the value of the object they are attached to. Auspicious eights are chosen for store openings, weddings, and signing dates. The single most famous example is the opening ceremony of the Beijing Olympics, deliberately set for 8 p.m. on 8 August 2008, a date stacked with eights to maximize good fortune.

The four-six-eight pattern shows the system whole. Four sounds like death and is shunned; six sounds like smooth flow and is welcomed; eight sounds like wealth and is prized above all. None of these feelings has anything to do with what the numbers count or with their dignified classical meanings. The whole of Chinese number luck rides on the sounds of the spoken language.

Two strokes parting, leaning away from each other: 八 is a picture of dividing, of one thing splitting into two. That is the character's true root, kept alive in 分 fēn (divide), where the same two strokes sit over a knife. The number eight only borrowed the shape.

Two things make eight unforgettable. In the classical world it is structure: 八卦, the eight trigrams, every combination of three yin-or-yang lines, the seed from which the sixty-four hexagrams grow. In the modern world it is fortune: 八 bā sounds like 发 fā, to prosper, and so eight is the number people pay premiums to own, the number behind an Olympic opening set for 8:08 on 8/8/08. A graph for splitting apart became the sound of getting rich.