9

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 3-look-alike.

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.

The Arabs simply connected that squiggle to the downward stroke at the middle and subsequent European change was purely cosmetic.

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.

[2] 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.

[3] If an odd perfect number exists, it will have at least nine distinct prime factors.

In the Vaisheshika branch of Hindu philosophy, there are nine universal substances or elements: Earth, Water, Air, Fire, Ether, Time, Space, Soul, and Mind.