It
is an unwritten law that to be Scottish, one must eat haggis every day,
have a west highland terrier as a "friend", say "hoots" and "och aye"
on an hourly basis, be able to sing every song from Brigadoon word perfect
and wear a tartan kilt (even in bed). However, tartan was originally
a style of cloth intended only to be decorative. These were patterns
that were popular within certain areas where they were woven, and local
patterns were achieved using the limited range of local colour dyes
(brambles, heather, onions and "midges" can only give so many colour
options). They were made of a local coarse type of wool which was guaranteed
to afflict the wearer with the infamous 16th Century ailment of "Scottish
Sporran Scritch Scratch". This reputedly afflicted thousands more in
Scotland than the Black Death.
Where there was a strong clan within an area (especially in the highlands where they got really scary!), visitors from other areas were recognised as being from a different clan by the pattern of their tartan. This caused problems when staying overnight at the local B&B (Bog & Bothy) if you inadvertently left your clan tartan "smalls" out and some local thief/idiot stole your only method of identification.
When tartan was proscribed by law, the Government or Black Watch pattern was the only legal one. But by the early 1800s, people realised that the knowledge of tartans before the ´45 was being lost and, simultaneously, everyone got "Romantic" about Scotland's past. They then felt it their duty to preserve all the known tartan designs. These were reconstructed from portraits, collected on pilgrimages (usually to the pub), demanded from clan chiefs, recovered from weaver's notes and special relics were regularly whipped out of granny's bottom drawer for inspection (usually on Saturday nights after Scotsport!).
The
significance of tartan as national dress, worn under various circumstances,
created clan tartans for every "name", even those that previously had
none. This was the first "cunning plan" by the Scots to milk a symbol
to death in the name of commerce. Hunting tartans of a subdued nature
(the equivalent of today's trendy combat wear) and dress tartans that
were brighter often supplemented these clan tartans. These inevitably
suffered overkill at the hands of such Scottish stalwarts as Andy Stewart,
Kenneth McKellar and the White Heather Club (otherwise infamously known
as the Tight Leather Club). However, this nickname seemed to refer more
to the profuse use of Badger Sporrans and leather G-strings by the male
dancers in the group!
Further variety was added by fashion, fancy or trade tartans to fill any niche, including various colours of a single pattern, especially the more universal designs used for dancing. There were even some "mourning" tartans developed (usually worn by Scots on days when England were due to play against such great football giants as Jamaica Under 9's and Scotland were playing Brazil in the first round - again!).
Nowadays, tartan is a marvellous way of re-establishing ones roots and proclaiming to society that you too are a member of a clan (well, we all want to belong, don't we?). TOP SECRET: Scots have been such prolific pioneers emigrating to other countries over the centuries that we now have huge underground groups and facilities throughout the world. These are ready to spring into action and we should don our kilts in the final world-wide take-over offensive (oops! must just pop off and see my analyst briefly). I hope this short and absolutely factual outline of tartan history answers a few of your questions.