I was tooling around on the net looking for information about the relationship of paper/cardboard "weight" to thickness, trying to find the thicker stock used to make centering rings and such. I came across a couple useful links to share (someone else asked about this sometime back IIRC)
The Card Modeling FAQ
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Paper Weight - Thickness
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(same info on another site)
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CJPW (Christopher John/The Printworks Inc) paper thickness guide
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And I thought the "gage" system for sheet metal was confusing!!