The 'heddles' made of wire or string, are the means by which the warp threads are lifted to allow the weft shuttles to be sent through the separated and lifted warp threads - it is this process which forms the tartan pattern. The 'tying in' of the warp threads allows the new thread pattern to be pulled through the 'heddles'.

'The Shuttles contain the weft threads with the appropriate colours, they are governed by a metal loop 'programme' which propels them across the loom in the correct sequence for that particular tartan.