the sequence of threads, known as the sett, starts at an edge and either repeats or reverses on what are called "pivot" points. in "diagram a", the sett reverses at the first pivot, then repeats, then reverses at the next pivot, and will carry on in this manner horizontally. in "diagram b", the sett reverses and repeats in the same way as the warp, and also carries on in the same manner vertically. the diagrams left illustrate the construction of a "symmetrical" tartan. however, on an "asymmetrical" tartan, the sett does not reverse at the pivots, it just repeats at the pivots. also, some tartans (very few) do not have exactly the same sett for the warp and weft. this means the warp and weft will have alternate "thread counts".