It should also be pointed out that the vast majority of "plaid" patterns you see on commercially sold products are not really tartans at all. And I'm sure someone will correct me if I'm wrong, but in order to be a tartan (whether it's registered or not), the sett must be the same in both directions (warp and weft). If you look closely, many "plaids" have different setts in each direction. It makes for a roughly-tartanish looking pattern, but not quite. It doesn't make square blocks of repeating patterns. I'm not sure why they do this.

It's pretty uncommon to see a true repeating pattern both directions in most commercial fabric, unless it's a simple check pattern. This isn't to say that there aren't plenty of actual 'tartans' out there in flannel or whatnot, but you'd be surprised at how many are not. Americans in particular will refer to anything that vaguely resembles a tartan pattern "plaid".