I'm likely going to come off sounding like a "know it all arse", but being Scottish, Welsh and Irish descent myself (and plenty of Prussian) and a Great Highland and Irish Uilleann piper, I HAVE to speak up:
1. A plaid is actually a cape like garment worn most often over the shoulder (although there are actually many ways to utilize this practical garment), regardless of the pattern or type of material it is made from.
In the context of Scottish Highland attire, it started out as an integral part of the length of cloth that was worn as a Great Kilt, the breacan feile, feileadh mhor, or philamore.
However, since the advent of the separate little kilt (phillabeg, or feilidh-beag) , it is now a disconnected piece in of itself.
2. The distinct patterned material with a cross running "sett", now associated primarily with Scotland, is actually called a Tartan.
Just to clarify. :)