亿

Chinese stacks numbers in four-digit blocks: 万 is ten thousand, and 亿 is 100,000,000. English stacks in threes: thousand, million, billion, trillion. That mismatch is why the two ladders never line up. So 一亿 is a hundred million, and 十亿 is a billion, the number you see in headlines. 一百亿 is ten billion. Anything bigger simply stacks 亿 again: 一万亿 is a trillion. When you read 'GDP破120万亿', that means 120 trillion, not 120 billion. Getting this wrong is a factor-of-a-thousand mistake, so it is worth drilling. “零百千万亿， 可以医椅衣。” (Zero, hundred, thousand, ten thousand, hundred million — they can sound like 'doctor,' 'chair,' 'clothes.') ── [《一二三（2）》](/read/counting-days-rhymes#s3)

亿 is a fourth tone. 一 never keeps its first tone in front of a fourth tone; it slides up to second. So out loud, 一亿 comes out yí yì, exactly the way 一个 is yí ge and 一半 is yí bàn. In front of first, second or third tones, 一 drops to fourth tone instead: 一天 is yì tiān and 一百 is yì bǎi. This is the [一 tone change](/grammar/yi-tone-change-35) rule doing its work automatically. As a plain digit, like 第一 or reading 1991, it snaps back to yī.

百万富翁 is a direct copy of 'millionaire'. Past that line, Chinese reached for 亿万, which literally reads 'millions and hundred-millions' and means fuzzy 'countless'. So 亿万富翁 is 'billionaire', and 亿万观众 is 'hundreds of millions of viewers'. Nobody runs arithmetic on it. Exact money talk stays exact: 身家过亿 means personal wealth past a hundred million, and 上亿 means over a hundred million. That phrase is the standard marker of real money in Chinese. Wang Jianlin's 'set a small goal first, earn 一个亿' joke only lands because 亿 is the entry ticket to that club.