As usual, paul has coverd most of the pertanant information. So I'll add just a few bits more.

The 9000m number was supposed to be the average length of the fiber (just one) a silkworms cocoon. I believe that (I read about this once, and just vaugly remember the numbers) the average silk is about 3d.

For a given type of nylon, it has approximatly the same fiber weight (about 5-7d for backpack type applications) thus the denier fo the material indexes into how many fibers per thread

: I've gotten various interpretations of "denier". At one time I
: understood it to be a measure of how many threads were woven in a given
: area. The more threads, the denser the fabric and the heavier it would be.
: That kinda made sense. I jsut found a formula describing denier at a
: website of a company which makes backpacks.

: http://www.lowealpine.com/Technology/Fabrics.asp

: According to this, 1 d (or one denier) is equal to 9000 meters of thread
: weighing one gram.

: Denier, therefore, relates to thread weight. So if you have 9000 meters of
: your thread weighing 500 grams, and you weave this into a fabric, then it
: will be a fabric that can be listed as being made from 500 denier nylon
: (or polyester, or whatever.

: Any thread that would qualify as jsut one denier is going to be extremely
: fine and very light weight. Any thread which measures 2000 denier is going
: to be a lot thicker, and probably a lot stronger and more resistant to
: breaking.

: What we are concerned about is the thickness and strength of the fabric, more
: so than just the thickness and strength of the threads used in making the
: fabric.

: Of course you can weave this thread densely to make a heavy weight cloth, or
: you can weave it loosely to have a more open, lighterweight cloth, and
: both types of cloth could be described as being made from 500 denier
: fabric. The densely woven fabric would have MORE threads per square unit
: (inch, yard, meter, whatever) and would therefore have a higher weight.

: So, the higher weight fabric made from the higher denier number threads give
: you maximum strength. Of course, this might be heavier and stronger than
: you really need, and more expensive, too, so buy according to your needs.
: Nine ounce fabric seems to suit most people, with some going for heavier
: material when they expect river snags, and others going for lighter
: fabrics when they want to build ultralight boats.

: Hope this helps

: PGJ