All of the above that people have said is largely true. I will only add that it isn't even really "thread count" that defines a tartan (as Kid said), but really the overall design. I'm not faulting Kid for saying that at all, as it is a mostly true statement. But sometimes you will encounter some minor variation in the thread count, and usually that's just fine.

Your tartan here is a case in point. Of course the largest difference in the cloth between the two mills is the color. The mystery mill on the right is obviously quite lighter in tone than the cloth produced by Strathmore. But there is more difference than that. Some of the proportion has changed.

If you look at the main yellow pivot stripe in the Strathmore sample, you will see that the black and blue lines that border it are the same width. If you look at that same stripe on the right sample, you will see that the yellow stripe is twice as wide as the black and blue. Sometimes this kind of doubling of a pivot line happens as a result of a misread of the thread count -- sometimes thread counts are recorded with a half pivot, and sometimes they are recorded with a full pivot. If you don't know what you are looking at, you could mistakenly weave the tartan with pivots twice as large as they should be. The normal practice today is to record a full pivot.

I don't think that's what happened here, though because the other pivot (the wide blue portion of the tartan) is not doubled in proportion. No, I think the two mills just have differing opinions as to what would create a balanced looking tartan. And that's ok.

You'll also notice that the mill on the right used black instead of a dark blue for those portions of the tartan (maybe they used a very dark navy blue, but on my monitor it looks black). Maybe that's because they thought a black would provide more contrast to the lighter colors? Who knows? Substituting black and dark blue is not unheard of in the tartan world (look at the two different MacAlpine tartans, for instance).

My point is that while the above differences represent changes in the tartan, they are minor changes does for aesthetic purposes. Anyone familiar with the Cian tartan would still look at the pattern and recognize it as the Cian pattern. Variation such as this from one woolen mill to another is actually quite common in the tartan world.

And it's not necessarily a bad thing, because it gives the consumer/wearer of the tartan some choice. And it is how the mills compete. Why would you wear the House of Edgar's tartan over Lochcarron or Strathmore or Marton Mills or any other mill? Because you like the way they produce your particular tartan. Someone else in your clan may prefer Lochcarron's cloth. And that's just fine. It lets people have some choice.

Now, a question you might have is, "Which one does the clan use?" Well, if you go to the clan web site, which is http://www.clancian-carroll.com/ (warning, it plays an audio file when you load it), you will see that the image of the tartan they show is like the one on the right. That's no surprise, as you bought that cloth from the clan! (I also note that if you go to the clan gift shop, they don't seem to have any tartan items for sale listed).

On the other hand, if you look at the STA records, the Cian tartan that they have listed matches the Strathmore sample. Looking at the STA notes, they say they have their woven sample from D. C. Dalgliesh. That means that at least two woolen mills have woven the tartan this way!

I also went back and looked at the old Scottish Tartans Society records (they originally recorded the tartan), and note that their image of the tartan looks like the Strathmore sample except that it appears to use black instead of a dark blue. However, when I went and looked at the actual thread count instead of just the image, I saw that it called for Navy Blue, not black, so here it was a case of the navy blue just looking rather dark.

Of course, a really big question is what is available? To the best of my knowledge, the only woolen mill producing this tartan in a kilt weight is Strathmore. This will, of course, limit your selection! And it also means that most people in the clan out there who have kilts will have them made from this cloth.

As for the other sample, do we know what mill the clan bought their tartan from? Do we know if it was stocked, or was it a special weave for the clan? And did that mill produce it in a weight heavy enough for kilting?

And, of course, the most important thing is, what version do you like?
Aye,
Matt