When associated with Scotland, it becomes the "tartan". Usually, one considers a plaid to be defining a Scottish clan although states, provinces, groups and people can have their own plaids. Check out this database for a list. It seems comprehensive, but I'm willing to bet some odd-ones are left off, particularly when it's a tartan created and registered for one person.
It used to be that you could not wear the tartan if you were not associated with the clan to whom Lord Lyon approved it. As Scotland became more aware of selling itself, this restriction was eased. Now, if you are a Schmidt, for instance, you, too can wear the Irn Bru Modern tartan simply because you like how it looks.
The above and to the right plaids aren't representative of anyone, to the best of my knowledge. The above was found in the box with the red cotton. I don't remember what I was going to do with either of these. There's more of the blue to the right than there is of the blue above.
There used to be kilt patterns you could buy from the major pattern outlets; Simplicity, McCalls, Butterick or Vogue. A quick Internet search doesn't show that they have these anymore. There are other places where I could get a pattern to make my own kilt but I don't have anywhere near enough material to do that. I'm thinking the above was going to be a vest. I don't know about the blue to the right.
If I had more of each, I would do more research into making kilts. Both lend themselves to that or to the kilted skirt which, technically, a lady wears. The kilt is, technically, for a man. Yeah yeah. I'm not splitting hairs either if you want to wear a kilt and you're female.
I put these back in the bottom of the box to await inspiration. Clan Mija and Clan Pilchard, maybe?