999 explained.

999 in everyday use

Nine hundred ninety-nine is the emergency telephone number in the United Kingdom, introduced in London in 1937 as the first such service in the world and chosen partly because the digit 9 could be dialled on a rotary phone in the dark by feeling for the finger stop. Ireland and several other countries adopted it too. On bullion, 999 marks gold or silver that is 99.9 percent pure, and 999-fine bars are the common investment grade. Prices ending in 999 are the largest form of charm pricing.

The number is 10³ − 1, and the recurring decimal 0.999… is exactly equal to 1, a fact that surprises many people and follows from 1/3 = 0.333… multiplied by three. It factors as 3³ × 37, with eight divisors, and 1/999 = 0.001001001…, which is why dividing by 999 repeats any three-digit block: 123/999 = 0.123123…. It is 1111100111 in binary and 3E7 in hexadecimal. Its digit sum is 27, which divides it, so it is a Harshad number.