Quote Originally Posted by figheadair View Post
Whilst most tartans are symmetic there are a number of old patterns that are asymmetric, Buchanan and Glenorchy being the two most common. In Britain, and probably elsewhere, they are known as asymmetric tartans.
Yes, they are asymmetric on the horizontal and vertical, in that the sequence repeats but does not reverse. That's what most people mean when using the term "asymmetrical" for these.

But the horizontal sequence is the same as the vertical sequence, making them symmetrical on the diagonal. Nobody even questions whether the horizontal and vertical sequences are the same or not; they always are in tartans.

In the U.S., there are lots of variations in fabric patterns where the horizontal sequence is not the same as the vertical sequence at all (and may not even be the same colors let alone order). These get referred to in the US as "plaid", but not "tartan".

Notwithstanding Nick's comment, the term 'plaid' is not usually used here in the UK, or in Europe more widely, to mean tartan.
Interesting. For my curiosity, what would "plaid" be used for, and what term would be used for a fabric pattern with different sequences in the horizontal versus vertical?

I find it interesting how words change and mutate over time and distance.