Sett size gives the formula that is made of thread count and colours used to make a tartan. once the formula is sett and registered, IT CAN NOT BE CHANGED and be what it was named as, to change the formula or sett, you will make a completely new tartan. the only way the set is changed is the ounce of the thread used to make the tartan. a 10 oz is smaller, therefore the tartan sett size is smaller then aa 22 oz thread and so on...there is NO OTHER WAY TO change the sett size for a given named tartan. hope this helps you. You can always create a new tartan based on an original tartan, by change in the number of threads per colour and repeat the pattern, but if it is to close to the original tartan the Registry of Tartan, may not pass if for registration.