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9999 — the number of ultimate completion
9999 is the last four-digit number, the boundary between the four-digit and five-digit world. In numerology: four nines mean the completion of all cycles at all levels. Sum of digits: 9+9+9+9=36, then 3+6=9 — nine again.
Numerology
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9999 rooms of the Forbidden City
According to legend, the Forbidden City in Beijing has 9,999 rooms — one less than 10,000, so as not to surpass the celestial palace of the gods. In reality there are 980 buildings, but 9,999 became a symbol of China's imperial greatness.
Culture
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Four-digit counter overflow
The Y2K problem was a consequence of 2-digit counters. Similarly, 4-digit counters 'overflow' at 9999: old car odometers, mechanical counters, game scores. After 9999, 0000 always follows.
Science