Something I don't know about after all these years...

[MacMillan of Rathdown]

The historical tradition is that you "belong" to your father's clan, ie: the clan most closely associated with your surname. So, while I have McBain, Boyd, Stewart, Chisholm, etc. cousins, my clan is Clan MacMillan. Could I wear the other tartans? Sure, in the same way I could wear any tartan I wanted-- but I don't. In the instance of what have come to be called "sept names" a degree of guesswork sometimes comes into play, especially in the absence of any genealogical evidence. Take the name "Bell"-- it can be a Highland name (from around Loch Tay), or a Lowland name (the Blackett House Bells from the Scottish Borders) or an English name. Without some genealogical link to a specific area a "Mr. Bell" who wishes to assert his believed Scottish heritage will usually turn to Black's list of Scottish surnames (the basis of most "sept lists") and from there "adopt" his tartan, and by extension become a member of a clan, wear the badge of it's chief, and take part in all of its activities. And in my opinion there is nothing wrong with that.

As Jock Scot has pointed out in the past, "multiple affiliation kilting" is a North American phenomenon not normally seen in Scotland where clan identity is more of a given. "You are what you are" being a very Scots attitude, very few Scots would think of wearing any tartan other than that directly associated with their surname or, perhaps, their regiment.