Finding the Pivots
Cloth is woven by threads at right angles to each other and, using a set of coloured threads in the same order, these combine to create a tartan pattern. Tartans are therefore woven on long coloured threads called the warp and the same pattern is then crossed through the warp threads, thread by thread, to form the weft, the weft threads passing twice from side to side back to the same edge of the cloth called the selvedge. The thread count is repeated in one of two ways. If repeated in reverse order, the tartan becomes symmetrical. If repeated from the beginning, then it becomes asymmetrical.
The Robertson pattern below is symmetrical whilst the Hebridean pattern is asymmetrical. In the Robertson the green tramline marks the end of the threadcount, which then goes into reverse. In the Hebridean, the dark stripe at the end of the count is followed by the green again. Whatever form of repetition, the tartan's pivots are the beginning and the end of the thread count but in an asymmetrical tartan, there is just one pivot. To locate the pivots, find a diagonal of squares that are all pure colour. Most tartans are symmetrical and the warp and weft cross to make these pure colours: the thread count becomes these boxes of colour in two dimensions. In some patterns, there are isolated squares where the same colour is used in different parts of the count but usually these are not square but rectangular. The Borthwick Tartan below shows a clear diagonal from top left to the magenta "cross". Having found a main diagonal, the pivots will be at the centre of two of them. Often these pivot under checks are embellished with an over check to pinpoint the pivot. In the Borthwick Tartan the pivots are visible as green and magenta "crosses". To find the pivots therefore, try to locate a diagonal made up of pure colour squares. Work out if the pattern reverses, is symmetrical, or starts again. Then identify the under checks, two of which hold the pivots.