The oracle-bone 九 draws an arm sharply bent at the elbow — some scholars read it as a hook, others as the coiling tail of a creature. Whichever it first pictured, its sound was borrowed to write the number nine, and that single bending stroke has stood for the highest single digit ever since.
A piece of culture
Nine: the emperor's number Nine (九) is the most yang, most powerful number in Chinese numerology — the highest single digit, doubled into 九九 to suggest "forever" (its sound echoes 久, jiǔ, "long-lasting"). The Forbidden City was said to have 9,999.5 rooms, one short of heaven's mythical 10,000, since only heaven itself could own a full ten thousand. Emperors were linked to the "nine-five" position from the I Ching, dragon robes were embroidered with nine dragons, and the Double Ninth Festival (重阳节) falls on the ninth day of the ninth lunar month — two nines together, doubly auspicious.