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999 — pricing psychology
Prices ending in 9 (999, 9.99, 19.99) are perceived as significantly less than round numbers. This technique is called 'charm pricing' and was introduced in the US in the late 19th century to force cash register receipts to be printed.
Culture
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999 in British emergency services
In the United Kingdom, 999 is the emergency services number (equivalent to American 911). Introduced in 1937 as the world's first unified emergency number. It receives over 30 million calls per year.
Numerology
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999 — gold purity
999 fineness (or 99.9%) means virtually pure gold — 'three nines fine'. This is the highest standard fineness for gold bars and coins. Absolutely pure gold (1000/1000) is technically unachievable.
Science