The official sett of a tartan , I guess, would be the recorded thread count with the tartan authorities. A repeat would be from one point in the sett to the exact same point in the next sett (middle of your yellow line to the middle of the next same yellow line. The thread count of the tartan may go something like 10K 5R 10K 5R 2Y 5R 10K 5R 10K. K=Black, R=Red, Y-yellow. The sett begins and ends in black. The yellow strip is in the middle. So a repeat might be the yellow stripe in one sett to the yellow stripe in the next sett, or the fist red stripe in one sett to the first red stripe in the next sett.

The size of the sett would be measured from the first black thread to the last black thread, in the above mentioned ,made-up, tartan. But you should get the same measurement if you measure from the middle of the yellow stripe to the middle of the next yellow stripe. The size of the sett is mostly related to the weight of the fabric. An 11oz tartan will have the same thread count as a 16 oz tartan. But the 11 oz might have a 5 inch sett, while the 16 oz has an 8 inch sett. The difference is that the thread used in the 16 oz material is thicker than the 11 oz thread.

When measuring a sett to determine it size measure along the length of the material not the width. I can't remember which is warp and which is weft. Sometimes setts may not come out as perfect squares. So meaure by the length since that is how you will pleat it.

All of this is explained the Barb T's book, "The Art of Kiltmaking". This is a must read for anyone interested in making a kilt. It is a worthwhile investment.

I hope I have stated everything correctly. It is approaching 1AM.

PS If you do have material that has a 3 inch sett, you can always consider pleating to every other sett. If you pleat to a yellow stripe, pleat every other yellow stripe. You will use twice as much material in the pleats, but your pleats will be twice as deep.

Happy Kilting