McTavish,

When you speak of direct descent from multiple clans, I assume you mean that six of your great great grandfathers all came from different clans- or possibly that six of your great grandfathers and great grandmothers all came from different clans. I believe, strictly speaking, that you would wear the tartan of the clan with a direct male line- your father's father's, etc.

We have gotten pretty egalitarian in the US about matrilineal descent and that is a fine thing- after all, matrilineal descent is more easily verified than patrilineal, absent a DNA test. But clan tartans and "the right to wear them" (assuming you believe such a right exists) are inherited in the male line.

Like primogeniture, the old rules are strict, but simple. Either your father and his father and his father hail from Clan X or you don't really have a claim to that tartan. Not your grandmother, not your grandfather's mother, not the name of your dog, favorite eatery, or the play you were in back in college. As has been said above, you may wear what you like. I believe Father bill has said it nicely, as have others, here and elsewhere. You may affiliate with clan associations or ask permission of chiefs or just stare down people who question you. But if you want to follow the rule, the rule is simple. Grafting other ideas onto the rule is what complicates things.

Now there is a distinct possibility that your father's father's father is not among those six lines that you can claim. While the strict interpretation says "No Tartan for you, Mac," I believe the liberal interpretation would be to choose the tartan next closest, that is, the one with the most recent male ancestor. If you can't make that work, then go with the one that matches your eyes.