Well, first of all, there are no official rules. According to the Court of the Lord Lyon King of Arms:
"There are no strict rules on who has the right to wear a particular tartan... Wearing a particular clan tartan indicates that the wearer bears an allegiance to the chief of that clan... There is no official register of tartan. Records of designs are maintained by the Scottish Tartans Authority, Fraser House, 25 Commissioner Street, Crieff, Perthshire, PH7 3A Y. The Lord Lyon has no jurisdiction over tartan..."
Official or not, many customs have been developed over the centuries about the wearing of various tartans. I have Stewart ancestry, so I thought I would look up that tartan. I was surprised to find that there are different ones for Stewart of Appin, Stewart of Galloway, Stewart of Rothesay, Stewart of Bute, Stewart of Bute Hunting, Castle Stewart, Stewart Royal Family Tartan, and many more. I'll ignore the livery tartan for Jacky Stewart's Formula One motor racing team as I doubt that I can claim that one anyway.
Eventually I did find a tartan for just "Stewart," but I still am not sure if that is the appropriate one for me or not. I guess I need to spend some more time in the genealogy records to see just where my Stewarts originated in Scotland.
The fact that there are so many tartans listed makes for a fascinating study. Luckily, you can find a lot of information about tartans online. You may even find the one worn by your ancestors.
Start your search at these sites:
The International Tartan Index (published by the Scottish Tartans Authority) at http://www.tartansauthority.com/Web/Site/Search/searchintro.asp
The Scottish Tartans World Register at http://www.tartans.scotland.net/find_tartan.cfm
The Standing Council of Scottish Chiefs at http://www.clanchiefs.org/tartanfinder.html
To become completely immersed in Scottish culture, visit Electric Scotland at http://www.electricscotland.com. This is an enormous site with more than 8,000 pages.
Hmmm, I wonder how I would look in a kilt? Oh, never mind.