As a certified, card carrying, member of the Benevolent and Protective Order of Old Farts, I'll just throw in my tuppence worth. (At least they still had tuppences when and where I grew up!)

Flat tweed caps should match the tweed jacket (with leather trim) which in turn was matched by the waistcoat (vest) and the plus-fours. In short, a totally coordinated outfit. Worn by the gentlemen of my youth when they were in the Great Outdoors, doing such things as shooting innocent wildlife, buying horses and the like. (Most of these were Irish or Anglo-Irish. (My Father, an American living abroad often wore leather putees over his tweed trousers, but then allowances were made for his being a "Yank".)

For the Kilt wearers; I only remember the Balmoral or TOS, and the latter only for those in uniform.

For me, unless the person is a piper, I do not care for the Glengarry.

Thus endeth the rant