A few things would be good to keep in mind.

First, if we look at this issue through the eyes of history, it is a realtively new thing. People in the Highlands of Scotland have been wearing kilts for more than 400 years. They have been wearing tartan for much longer than that. We don't know how long tartan has been worn in Scotland, but archaeological evidence can be found from about 1700 years ago. Even older tartan specimens (some as far back as 5000 years ago) can be found in other parts of the world. For just about all that time, particular patterns of tartan had no meaning. People simply wore patterns and colors that were available in their area, that they could afford, and that they liked.

The idea that a tartan must have a name, and that it somehow represented the clan/family/district whose name it bore, can be traced back to the late eighteenth century at the earliest and really didn't start to gel into something formalized until the nineteenth century. When you look at this in light of the "great antiquity" that most people ascribe to these clan tartans, it is a shockingly new development.

There is ample evidence that people in the Highlands of Scotland, during much of the nineteenth century, who grew up with the tradition of kilt wearing, paid no heed to the notion of "clan tartans" and continued to wear any tartan they liked. I still know Scots who do the same. So I think no one should feel like they are breaking some sacred code if they follow the same tradition.

On the other hand, the system of named, representative tartans has been around for about 200 years. This is almost as long as the United States of America has been around! So it's more than long enough to firmly establish a tradition. Moreover, even the leaders of the clans themselves have adopted this system, selecting and approving tartans for their clans. It cannot be denied that most people who wear a tartan do so with the intent of wearing it in a representative manner. And most people assume that when others wear the tartan, they are doing the same.

Moreover, many of the tartans that we have available for us today have been designed during the past 200 years. Rather than being a traditional folk design that has been later adopted by some clan, these tartans were designed by someone specifically to represent this particular clan, family, district, group, corporation, etc. This was the reason for which they were designed, it is the name they are sold under, and it is why they are being worn. (There are also many tartans that have been designed during the past 200 years purely for fashion purposes). So all of this needs to be understood and taken into consideration.

In short, realize that there are no rules or laws proscribing what tartan anyone may or may not wear. This is a myth. Tartan is not like heraldry (the Lyon Court in Scotland actually operates as a court of law, and you can get into legal trouble if you display or use heraldic arms that are not rightfully yours). Some assume it is the same with tartan. It is not. So don't even think of it in this way.

But there are customs, conventions, and traditions that have developed over the past two centuries. And as I have shown above, some people (and I am talking about native Scots) take these traditions more seriously than others. So I advise people, when selecting a tartan, to choose any tartan you like, but know what tartan you are wearing, and have some reason for wearing that tartan. For most people it will be a familial connection. But there are other reasons one might have to wear a tartan. Just know that most people who see you, if they recognize the tartan, will assume that you are affiliated with what that tartan represents.

Last point -- keep in mind also that the modern notion that your clan is determined by your surname is completely ahistorical. What clan your anscestors belonged to had nothing to do with their surname -- most people didn't even have surnames for most of the clan histories. The clan you belonged to was largely determined by where you lived. If you lived on this clan's territory, you gave your allegiance to that clan's cheif and were considered a part of that clan.

Even today, in a strict sense, this is what defines clan membership. Strictly speaking, very few people who consider themselves "clan members" today can actually prove that they are descended from that clan. But they give their loyalty and allegiance to that clan's cheif (or if the clan has no cheif, to the clan in general). And this is what makes you a part of the clan, when it comes down to it. I have heard of people who have no Scottish blood whatsoever who have written to different clan cheifs, asking for permission to join their clan -- and they have recieved it! Now who would deny someone the wearing of a tartan if they have a letter from the chief saying they are part of the clan?!

I don't think that it's neccesary to go this far (though it is neat!) if one wants to give one's allegiance to a particular clan, feel free. As others have mentioned here, when you enquire with most clan societies (which are not the same as the clan itself, although many operate with the clan's approval), they will gladly accept anyone who wants to affiliate themselves with the clan.

I've rambled on long enough, but in short -- this is not a cut and dry issue. There are many considerations to take into effect. But there are enough reasons why one may wish to wear a particular tartan, none of which one can judge or determine simply by seeing someone in a kilt. In my not so humble opinion, it would be the height of rudeness to approach someone and challenge their "right" to wear such a tartan. As long as you feel you have a valid reason for wearing it, that should be good enough.

Aye,
Matt