why number 9 in kill -9 command in unix?

Posted by Alby on Stack Overflow See other posts from Stack Overflow or by Alby
Published on 2012-03-30T23:24:27Z Indexed on 2012/03/30 23:29 UTC
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kill

I understand it's off topic, I couldn't find anywhere online and I was thinking maybe programming gurus in the community might know this.
I usually use

kill -9 pid

to kill the job. I always wondered the origin of 9. I looked it up online, and it says

"9 Means KILL signal that is not catchable or ignorable. In other words it would signal process (some running application) to quit immediately" (source: http://wiki.answers.com/Q/What_does_kill_-9_do_in_unix_in_its_entirety)

But, why 9? and what about the other numbers? is there any historical significance or because of the architecture of Unix?

Thanks!

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