I am on a quest~!

[Woodsheal]

The daytime plaid isn't like a fly plaid at all. It's just a rectangular piece of tartan which is folded and layed on the left shoulder (a bit cumbersome I know). It is then unfolded and draped over the shoulders or pulled over the wearers head to protect him from the elements. It could also be laid down on the ground to be sat on rather like a picnic blanket. When worn with classic day dress I think it looks great.

The practice dates back to the days when the earlier "great kilt", or belted plaid, was split down into two components: the little kilt as we now know it, and a smaller shoulder plaid. The latter served as the Highlander's "overcoat" as freddie describes.
The shoulder plaid was a goodly-sized item, as it still served as a blanket at night, but was smaller in dimension than the old double-wide plaid. It was worn wrapped about the upper body, in "blanket roll" fashion: