9 (number)

Nine (symbol: 9) is a natural number that follows eight and precedes ten. It is the largest single-digit number in the base-ten decimal system, meaning that numbers greater than 9 are written with two or more digits.

Nine is a composite number, as it is divisible by 1, 3, and 9. It is also the square of three (3 × 3 = 9), and is the second square number after one. It is an odd number because it is not divisible by two.

Mathematical properties

Nine has a notable property related to divisibility: when the digits of any number are added together and the result is divisible by 9, the original number is also divisible by 9. For example: 234 → 2+3+4 = 9, so 234 is divisible by 9. This property arises because dividing 10 by 9 leaves a remainder of 1. The same method is used for the number 3.

When 9 is multiplied by another number, the digits of the result, when added together, typically yield 9 or a number divisible by 9: 9 × 2 = 18 (1+8=9), 9 × 3 = 27 (2+7=9), 9 × 7 = 63 (6+3=9). This pattern makes it easier to verify arithmetic done by hand.

Symbol and history

The modern symbol for 9 originates from Hindu-Arabic numerals, which spread to Europe through Arab educational centers during the medieval period. In Arabic script it is written ٩. The Roman numeral system represents 9 as IX, meaning ten with one subtracted.

Common usage

Nine appears in various measurements and systems: a clock face has 12 hours, though 9 is used as a basic unit in certain games. In Somali, the word 'sagaal' is the ninth number in the counting sequence; its ordinal form is 'sagaalaad'.