This is just a quirky function of our base-10 number system. It happens in any place-value notation of base n for the multiples of n-1.

Also, if you take the digits of multiples of n-1 and add them together, and add the digits of the result together, and keep adding digits until you have a 1-digit number, it will be n-1.

Eg. In a decimal notation, n-1 is 9.
9x3=27, 2+7=9
9x11=99, 9+9=18, 1+8=9
9x987=8,883, 8+8+8+3=27, 2+7=9

In a hexadecimal (base 16) notation, n-1 is F (in hexadecimal; "15" in decimal).
Fx3=2D, 2+D=F
Fx11=FF, F+F=1E, 1+E=F ("11" in hexadecimal is decimal "17")
Fx3DB=3,9D5, 3+9+D+5=1E, 1+E=F

In binary (base 2), n-1 is 1. So this works for any number at all.
10, 1+0=1
11,010,101; 1+1+0+1+0+1+0+1=101, 1+0+1=10, 1+0=1.
etc.

And, to think this so-called "New Math" was far to much for the American public school system. The "base" or "radix" of a number system flies over the heads of many people, and important mathematical concepts go right with it: Math is simply a language. Numbers simply nominate a quantity. Having a base or radix--ten fingers or "digits", 0 through 9--just makes it a hell of a lot easier to see it than Roman numerals did. (Thank you, Arabs.)

I suppose now might be a good time to poll on recommended reading pertaining to math and numbers.

I found this clock on-line, which has each number expressed with 3 nines:



Not surprisingly, there were idiots getting into the whole 0.999... (repeating) = 1 debate, because it uses that to mark Seven. Again, there's no debate. When the nines are going off to infinity, the value is exactly equal to 1.

A lot of the people who were debating otherwise were, also not surprisingly, computer programmers. They're an example of people who erroneously think of "infinity" as actual number. It isn't. There is no "last" or "infinities" place in the decimal. Fortunately there were others setting them straight.

Regardless, I think it's a cool clock.

Speaking of modulus arithmetic, in numerology there's the common practice of adding up the numbers corresponding to a person's name, then adding the digits of the result together, and repeating until you wind up with a single digit. Mystics still go through all of this, when all they really have to do is just divide the original number by 9 and take the remainder (or if it's 0, set the result to 9). But...don't tell them that!

Multiples of 9 are also the underlying reason why this old on-line mind-reading trick works:
http://www.bored.com/mysticalball/index.htm

You can thank the IEEE for that computer programmer infinity thing, or the fact that computer science in general has not the mathematical foundation it once did. That could be a chicken/egg scenario.

And now, for some REALLY misleading math with the number 9:

I might buy the sign for $9 just because. That is breathtaking.

Do you think anyone jumped at that deal?

I believe that they can get in trouble from the Dept. of Weights and Measures for that sign. If the original price was $29.99 and they are saving $-9, then the item would cost $38.99 not $39.98. That is a $300 fine by the DofW&M in San Bernardino County. They better fix it quick.

That definitely belongs on failblog.