fabric stores are dangerous places...

[Andrew Breecher]

I recently got some fabric of a different nature: I'm going to make a backpack for my next hike. Now I just need a source of thick (2-3mm) plastic for a framesheet.

Andrew.

[MacWage]

A tartan, as now understood is a pattern of criss-crossed lines that form a regular pattern and are the same on weft and weave. However, "Welsh tartans" often DO NOT match weft and weave. Orginally, tartan refered to the fabric itself, which was "hard" faced worsted wool in a twill.
Today, some only use the term for "registered" tartans.
Those "psuedo-tartans" are termed "bumbee" tartans, or "fashion tartans" (which can be either registered or unregistered, but don't have a specific clan association).

To understand tartan, know there are literally thousands of registered tartans and many are not easy to ID and may unitentially be copied by a well-meaning fabric mill. Also, that they are named onlt about 1800.
Mill manufactures used to number the patterns, but during the romanticism era and the days of Walter Scott's books and Burns poetry, they began to apply names to ID the patterns. Some names were adopted from regiments (like Black Watch and Gordon), while others were from large orders of a pattern (like MacPherson). Some were artifacts (like Culloden), while others were just random names added for a sense of romance. Clans then adopted the tartan bearing their names, or designed some for themselves or adopted the pattern of an old family heirloom.
THAT is the reason manny say, "wear what you like," but know what it means if it does mean something. I dub my own "bumbee" tartans and have "Jedi" (the one worn at Greenville Games) "Batman" (the blue and black one in the most recent material in a great kilt thread) and "Auld South." The author and Kilt maker Bob Martin did that to some material he had and someone wrote down the thread count and submitted it as Martin Tartan, which is now registered as such.

If I find a some material I like, but don't know if it is a real tartan, I buy anyways. If I can ID it, better. It is MUCH cheaper to experiment with that than Strome, but Strome makes a MUCH better looking and wearing kilt (especially in box pleats).