I understood that "tartan" referred to the weave - under 2 threads, over 2 threads. That weave gives the distinctive diagonals when 2 colors cross. Most patterns that you think of as plaid are over one, under one. Also, they're generally not symetrical. I know there were a few non-symetrical tartans, but they really seem to have been an exception, more than the rule, at least since Victorian times when tartans as we know them really came to be developed. I remember reading that the non-symetrical Buchanan tartan was really the result of a misprint in a book that it was listed in.