I’m new to TSD, so forgive me if I’m beating a dead horse. Or putting my comment in the wrong place.
I read Cecil’s answer to the kilt question in the archives, and would like to add to it, giving Cecil an actual source to cite.
According to J. Charles Thompson in “So You’re Going to Wear the Kilt” (3rd Revised edition, ISBN 0 86228 017 6), page 97:
"There is one firm answer – in Army uniform regulations, though I am told that these differ in detail from one regiment to another. A highland soldier in the kilt is out of uniform if he wears anything under it except when dancing, taking part in Highland games, or participating as a bandsman. If that last exception puzzles you, watch the way a band marks time with the knees coming up to waist height.
“The civilian may please himself.”