9 (nine) is the natural number following and preceding.

Evolution of the Hindu–Arabic digit

Circa 300 BC, as part of the Brahmi numerals, various Indians wrote a digit 9 similar in shape to the modern closing question mark without the bottom dot. The Kshatrapa, Andhra and Gupta started curving the bottom vertical line coming up with a -look-alike. How the numbers got to their Gupta form is open to considerable debate. The Nagari continued the bottom stroke to make a circle and enclose the 3-look-alike, in much the same way that the sign @ encircles a lowercase a. As time went on, the enclosing circle became bigger and its line continued beyond the circle downwards, as the 3-look-alike became smaller. Soon, all that was left of the 3-look-alike was a squiggle. The Arabs simply connected that squiggle to the downward stroke at the middle and subsequent European change was purely cosmetic. While the shape of the glyph for the digit 9 has an ascender in most modern typefaces, in typefaces with text figures the character usually has a descender, as, for example, in. The form of the number nine (9) could possibly derived from the Arabic letter و|waw, in which its isolated form (و) resembles the number 9. The modern digit resembles an inverted 6. To disambiguate the two on objects and labels that can be inverted, they are often underlined. It is sometimes handwritten with two strokes and a straight stem, resembling a raised lower-case letter q, which distinguishes it from the 6. Similarly, in seven-segment display, the number 9 can be constructed either with a hook at the end of its stem or without one. Most LCD calculators use the former, but some VFD models use the latter.

Mathematics

9 is the fourth composite number, and the first odd composite number. 9 is also a refactorable number. Casting out nines is a quick way of testing the calculations of sums, differences, products, and quotients of integers in decimal, a method known as long ago as the 12th century. If an odd perfect number exists, it will have at least nine distinct prime factors. 9 is the sum of the cubes of the first two non-zero positive integers which makes it the first cube-sum number greater than 1|one. A number that is 4 or 5 modulo 9 cannot be represented as the sum of three cubes. There are nine Heegner numbers, or square-free positive integers n that yield an imaginary quadratic field whose ring of integers has a unique factorization, or class number of 1.

Geometry

A polygon with nine sides is called a nonagon. A regular nonagon can be constructed with a regular compass, straightedge, and angle trisector. The lowest number of squares needed for a perfect tiling of a rectangle is 9. 9 is the largest single-digit number in the decimal system.

List of basic calculations

Alphabets and codes

Culture and mythology

Indian culture

Nine is a number that appears often in Indian culture and mythology. Some instances are enumerated below.

Chinese culture

Ancient Egypt

European culture

Greek mythology

Mesoamerican mythology

Aztec mythology

Mayan mythology

Australian culture

The Pintupi Nine, a group of 9 Aboriginal Australian women who remained unaware of European colonisation of Australia and lived a traditional desert-dwelling life in Australia's Gibson Desert until 1984.

Anthropology

Idioms

Technique

Religion and philosophy

Christianity

Islam

There are three verses that refer to nine in the Quran. "We surely gave Moses nine clear signs.1 ˹You, O Prophet, can˺ ask the Children of Israel. When Moses came to them, Pharaoh said to him, “I really think that you, O Moses, are bewitched.”" Note 1: The nine signs of Moses are: the staff, the hand (both mentioned in Surah Ta-Ha 20:17-22), famine, shortage of crops, floods, locusts, lice, frogs, and blood (all mentioned in Surah Al-A'raf 7:130-133). These signs came as proofs for Pharaoh and the Egyptians. Otherwise, Moses had some other signs such as water gushing out of the rock after he hit it with his staff, and splitting the sea. "Now put your hand through ˹the opening of˺ your collar, it will come out ˹shining˺ white, unblemished.2 ˹These are two˺ of nine signs for Pharaoh and his people. They have truly been a rebellious people.”" Note 2: Moses, who was dark-skinned, was asked to put his hand under his armpit. When he took it out it was shining white, but not out of a skin condition like melanoma. "And there were in the city nine ˹elite˺ men who spread corruption in the land, never doing what is right."

Other

Science

Astronomy

Chemistry

Physiology

A human pregnancy normally lasts nine months, the basis of Naegele's rule.

Psychology

Common terminal digit in psychological pricing.