Scottish clans, historically, were a system of tribal groups, a system which can be traced back to around the 12th century. These clans were led by a chief, who had unlimited power and authority over its members. Clan members bear the surname of their chief, whether related by blood or not.
Historians believe that clans evolved in the first to be warriors, with the primary intention of protecting the herds. Clan mottos, such as that of the Stuart clan, "no one harms me with impunity," seem to provide credibility behind this theory. Ancient clans were also quite regimented, creating rules for behaviour, which spurred the reputation of the noble code of Scottish hospitality, which lives on today.
Clans were, even back in the 12th century, known to wear a chequered wool blanket-like cloth, which was called a plaid. In early times, the plaid was worn similarly to that as a Roman toga, wrapped, folded, and pinned in place. By the early 17th century, the cloth became a bit thinner, and was called "tartane" ("light wool"). The tartan was belted, the ensemble was known as "Feileadh Mor," or "great plaid," and this was wrapped around the waist and shoulders, over other clothing.
With this early progenitor of the kilt, a bonnet was worn. The bonnet was symbolically quite important, as it identified the clan leader where necessary, and clans were distinguised by a differing plant badge worn at the front with the clan crest, especially on the day of a battle.
A sporran traditionally was made of goat or badger skin, and hung in front, just under where the tartan was belted. Since the Scot had no pockets, this pouch acted as a purse, which had several compartments. Typically a man would use the sporran to hold possessions such as his watch, money, shot for his guns.
When travelling, a knapsack called a dorlach was carried on the right side at times, which would hold food items and other provisions needed for longer journeys. A "dirk," which was basically a knife blade about a foot long in length, would also have been worn, to complete the ensemble.
The Kilt Grows Up
After the defeat at the Battle of Culloden in 1746, all clan land was entitled to Royalty. In addition, clans were banned of wearing tartan for nearly 100 years. Anyone caught wearing it faced a heavy penalty - that of being sent in exile for up to seven years!
Once the ban on wearing tartan was lifted, there was much renewed interest in Scottish heritage. People started to wear a pattern of tartan that was common to the area in which they lived, so tartans began to become identified as much by area as by family.
The modern clansmen dress today is mostly reserved for formal occasions, and those engaged in tradional activities (ever see a piper wear jeans?). The one piece feileadh mor has been replaced by the more practical feileadh beag ("small plaid"), which is about 23 feet of material with a double apron at the front fastened by a pin. The modern kilt is reported, ironically, to have been invented by an Englishman, Thomas Rawlinson, to keep his Highlander employees from harm in his iron works factory.
Whether or not clansmen wear knickers under their kilts is a complete mystery which should probably remain that way. If you're dying to know, though, perhaps you can find out on the following informational web sites: