Yup - remember that sett (which has two "t's", not one) is the precise _proportions_ and sequence of colors in a tartan. Another way of thinking about it is that it is the minimum number of threads of each color that completely defines the tartan.

If you weave the tartan with the minimum number of threads (e.g., 2 black, 16 red, 16 black, 2 yellow, 16 black, and 16 red for the Red Wallace, a very simple tartan), it's easy to see how the weight of each thread would make the size of the sett bigger with the heavier threads. (Oh - and remember that the minimum number of threads in a color block is two.) But! Because sett refers to proportions, rather than absolute numbers of threads, you could imagine having a lightweight tartan with 8 threads of black, 64 threads of red, etc. that would have the same proportions but a bigger sett than one woven with heavier threads.

I have a couple of samples of tartans at home where the 13 oz version has a bigger sett than the 16 oz version of the same tartan. As Matt says, you need to ask (all mills that I've dealt with are very good about providing sett size, if you ask) or get a sample. I often ask so that I can judge how much tartan to order.

Cheers,

B