Tartan can initially appear complex in design but is based on a simple concept - a pattern of stripes which repeats in both a horizontal and verticle direction - in weaving terms the 'warp' and the 'weft'. A sequence of stripes (the sett) starting at one edge is repeated but in reverse order round a 'pivot' point. The sett reverses again and repeats round a second pivot point - see diagram 'A'. This process continues for the width of the material in question. The same sett and mirroring sequence occurs in the weft - diagram 'B' - continuing for the entire length of the material. The resulting combination of warp and weft is the tartan - diagram 'C'. (Note that the pivots themselves are not doubled in the mirroring process).
Illustrations by Don Pottinger
Some tartans - known as 'asymetrical' or 'non mirrored' do not reverse the sequence at the pivot points but simply repeat the sett in the same sequence. A very small number of tartans have different thread counts for the warp and the weft.
A description of a tartan can be written in a format known as the 'thread count' or the 'sett'. This identifies the colour and width of the stripes between two pivot points and incorporates the same information for the pivot points themselves. It takes the form of e.g. B16, LG8, K4, G24, K6 - reading the sequence from the left. The outside threads (B16 and K6 in the example given) are the pivot points. The colour is determined by a one to three letter shorthand - 'K' for example is black, 'B' is blue, 'LG' is light green and so on. The number after the letter is the proportional measurement - it does not matter what unit of measurement is used as the actual tartan pattern will remain the same - reflecting the abstract nature of tartan design. Traditionally the number refers to the number of threads used to set the loom but this can of course vary depending on the type and weight of thread used - modern tartan fabric can be made from silk, wool, cotton, poly-viscose, etc. In addition tartan designs are now used extensively on non-woven materials - paper, plastics, packaging, wall coverings, etc. although some claim that a tartan needs to be woven before it can be considered a 'true' tartan.