My thoughts on the subject of the fly plaid are thus. I believe if not mistaken that the fly plaid came about only as a modern version of the old Great Kilt or Feile Mor as the "Fashion" of wearing the Kilt and Tartan was revived in the 19th Century. It was an invention so to speak. When and where regulations were placed on the fly plaid and its wear is a question for the experts if even they could answer that one. I do think that the wearing of the fly plaid with a tee shirt or other attire is quite ridiculous, I dont knock anyone for its wear with a Tweed Kilt Jacket whether or not it be a "formal" occasion such as a wedding. The Macdonald in the above picture from the late 19th Century seems to be wearing one, and does not look like he is headed to a wedding anytime soon unless he expects trouble. Just a thought.