I am a Brit by nationality and I am a US patent agent. As such, I am qualified to advise on US utility patents (regular patents, not utility models, which means something else in certain countries) and on US design patents, but not on copyright or trademark, except when advising which type of intellectual property is appropriate, and cannot advise on anything relating to prospective litigation. However, unless I am advising a specific client on their specific legal problem it doesn't count as legal advice anyway, but I have to disclose that it's not legal advice. That was a disclaimer.

Design patents in the US and design registrations in the UK, as well as similar things in other countries, often called design models, protect the ornamental appearance of an article for a set period of time, 14 years in the US and the UK. There are some designs for tartans, but once they expire there is no more protection. Also, although they are colour in the UK, in the US they are black and white!

Utility (i.e. not design) patents last 20 years from filing, and they protect the functional aspects of things, the way they are made or the way they work, summing it up as simply as possible, but they must either be novel (new) or non-obvious (which in European law is termed having an inventive step). You can get a patent for a fabric, or a fastener or even perhaps features like pockets if they are really different, but you can't patent a tartan.

Trademarks protect the use of a name in commerce, and can be renewed indefinitely. However, a trademark would only protect the name of a tartan, not the sett, and to get it trademarked you have to make and sell it.

Copyright covers artistic works, and will protect your tartan sett long after you are dead. Seriously, it used to be 50 years after death, but was extended to 70 (at least in the US, not sure if it's 70 or still 50 in the UK) specifically to keep Mickey Mouse out of the public domain. Really, I'm not joking.

However, the UK, where most of the mills are, has no copyright register. In practice this tends to mean that they need to lodge their sett with some independent register to prove the date, and tartan registers tend to require a woven sample, as Mr. Newsome pointed out.

The US has a copyright register, but to comply with international treaty, copyright has to come into existence when the work is created, not when it is registered. Their way around this is to require you to register only one day before you sue, although you can register earlier if you choose to. This means, however, that although you can search the US copyright register, there is no guarantee that any particular copyrighted work is actually in there. It may pop up the day before they sue you!

Another quirk of copyright that is not found in any other form of intellectual property is that a foreign copyright is all that is needed to sue you. In comparison, you must have a patent, design or trademark in the country where you want to sue. Not so with copyright.

So for example, if a UK mill has proof of their UK copyright, such as, but not limited to, an STA registration, that is all they need to have a cause of action against you in the US. In case that still isn't clear, they don't need to have a US copyright to come after you in a US copyright case, as long as they have copyright somewhere.

OTOH, you have no cause of action in copyright unless you have been copied, which is not true of other types of IP. If you have a design for your sett (which covers the country they are in and hasn't yet expired) you can sue even if they had no knowledge of it, for example.

This is not legal advice, believe it or not, and you are not my client.