Most Black Watch variants, when pleated to the stripe, will be pleated to the stripe which appears twice in each sett. This is because the Black Watch pattern is so large. Often when mills weave it the sett size is on the order of 14" or more (though this of course will vary).

So, for example, this would mean the MacKenzie tartan pleated to the white line, the Gordon to the yellow line, the Robertson Hunting to the red line, etc.