Why are some older tartans like Stewart or Black Watch universal? (Not trying to stir up controversy, if Stewart's not a good example pick another name, just want to learn.)
I have always understood that the Government Sett, also known as "Black Watch" (not Blackwatch) was a "universal" tartan in that at one time, the majority of Scottish regiments wore some version of it, and/or was the basis for almost all of the regimental tartans. So, the "universal" refers to its use by the British Army, and not as a "universal" tartan worn by all. Of course, modern-day marketing has made it a "generic" tartan, along with the Stewart Royal, which technically is also a military tartan, and worn only with permission of the Sovereign, although again, due to the tartan trade, it has become a "generic" tartan.

Cheers,

Todd