There are many tartans closely connected with the Royal Family which are
worn fairly widely, but others, in particular the Balmoral shown below,
should never be worn by anyone outside the Royal Family.
Like everything else in tartan, it is convention that controls its use.
With Royal Tartans this is strictly observed. Exceptions are the Royal
Stewart and the Earl of St Andrews which have been greatly used commercially.
The latter has become the St Andrews District tartan but the former is
still for Royal use and that of pipers of Royal Regiments.