Knotty, the tartans you refer to on the Scotweb site are dance tartans which are only woven in 10/11oz material as standard by many of the mills (you could get a heavier weight by special order from somewhere like Dalgliesh). There are a lot of tartans where different hues are used for dancing purposes. A very good example of this is the Menzies tartan which from the last count I did was 7. Hope this clarifies some of the mystery