The clan system was one of the characteristics found in nearly all of the diverse peoples of the Celts. It consisted of a kin or extended family, all of whom traced their ancestry back to a common ancestor. The chief inherited his authority from this "founder" , and was an absolute authority to whom all members owed complete loyalty. He in turn was obligated to take care of them and lead them for the common benefit. The chief would refer to the other members as his clan, from the Gaelic word clann, meaning children. A clan would usually control a certain territory, including a village and surrounding farmlands. The chief was usually considered to own everything and provided for the rest of the clan according to his own judgement. He could also initiate temporary alliances with other clans or, more often, on raid to steal their cattle. (What would get you hung for rustling in the American Old West was considered a highly honorable sport among the early Celts. You could still get killed for it, but at least you were respected.) Most Celts looked with great suspicion on the idea of any authority above that of the clan chief, a fact which kept them from ever uniting into a nation which could resist the power of Rome and the Anglo-Saxons and Normans.
In the Highlands of Scotland, the clan system was particularly strong, probably due to the harshness of the environment. The Highlands are mostly composed of hills and rocky ridges, with relatively fertile valleys or straths between them. Even in the straths, there is limited land suitable for farming or grazing livestock. The cold climate and fairly short growing season also limit the productivity of the land. Under these circumstances, some sort of organized effort was almost essential and the the clan system was an ideal solution. A clan would settle in a strath and, if all went well their joint efforts could wring a living from the land. The terrain and the effort it took to survive tended to isolate them and make the all the more dependent on each other. What contacts they had with other clans were very likely to be raiding, or defending themselves from raids by others. Making the proverbial virtue of necessity, they came to take pride in their hardiness and to feel superior to others, including their own countrymen in the more hospitable lowlands.
In time, some clans became relatively large and powerful and began to absorb other groups who did not trace their descent to the original founder. In many cases, these other groups were more distant relations or allies by marriage. There were also cases where a smaller clan might petition and be accepted for adoption by a larger, which would benefit both groups. These adopted groups were called septs and would maintain a certain identity within the larger clan. They might retain their clan name and even have their own chieftain, subordinate to the Clan Chief.

Most peoples have no sooner discovered woven cloth than they are seized with the desire to decorate it. One of the most basic ways of doing this is to use various colors of thread in the weaving, producing stripes or checks. The Celts, with their love of decoration and color certainly adopted this custom early on. Among the Scottish Highlanders, certain patterns came to be identified with individual clans, affirming the members' pride in their identity and allowing them to recognize one another in battle. The basic pattern is called a sett and defines the color, width and sequence of the horizontal and vertical stripes which are repeated throughout the cloth. Septs often had their own tartans, although they were also entitled to wear that of the larger clan. Tartans certainly go back at least to the 15th century AD, and there is some evidence that the Celtic Scotti had them when they originally settled in Scotland.
In 1746, the Battle of Culloden marked the end of Scottish independance and the suppression of the clan system by the victorious English. The wearing of distinctive Scottish dress, including the tartan, was prohibited on pain of death. This act was repealed in 1782 but the sweeping changes in Scotland had ended the traditional power of the clans by that time. Ironically, the resurgent popularity of tartans and kilts is probably due to Queen Victoria in the nineteenth century. The Queen was proud of her Scottish ancestry and did much to popularize the traditions.
In modern times, many clans have a bewildering variety of tartans identified with them. A clan with a rather conspicuous sett might adopt a darker, more subdued one for hunting and perhaps raiding; these are known as Hunting Tartans. In addition, with the introduction of chemical dyes, colors became brighter and these are called Modern Colors. In some cases, the clan might still retain the sett of the older vegetable dyes and these are referred to as Ancient Colors. There are also Muted Colors, in which an attempt is made to duplicate the Ancient Colors with modern dyes. Finally, some clans have Dress Tartans which usually involve the substitution of white as the background color.
Today, a few purists maintain that only those descended from the historic Highland Clans have a legitimate right to wear the tartan. The majority view is that any family or group can have one, although it's considered questionable taste for non-members of a clan to wear that clan's established sett. Cities, states, counties and military organizations have all adopted and registered their own tartans. All of the Irish counties and several Irish family names are registered. The oldest of the latter is Murphy, which goes back to about 1800; the Murphy tartan (at the right) is also called Tara and is considered appropriate for anyone of Irish descent. There is an official registry in Scotland which will validate any acceptable sett, and there are services available to create designs. At the 1998 Rio Grande Valley Celtic Festival, some of the clan societies presented a tartan which they had had designed and registered for the State of New Mexico.

Fundamentally, a kilt is a strip of cloth wrapped around the body, an idea that has probably been used by a majority of peoples at least some time in their history. Actually the Celts are more usually pictured wearing trousers, which they discovered early in their development. The Scots and Irish however continued to wear the kilt at least part of the time and it developed a distinctive form in the Scottish Highlands. The standard width of woven cloth there was about a yard and the traditional kilt, later called the Great Kilt, was made of two strips of woolen cloth each 4-6 yards long. These were stitched together the long way creating a rectangle 4-6 yards long and 2 yards wide. In use, the cloth was hand pleated perpendicular to the seam, leaving an unpleated apron long enough to cover the front of the body at either end. The kilt was belted around the waist so that it hung to the knees, with the aprons overlapping in front and the pleats going around the sides and back. The material above the belt was grasped about the middle of the upper edge, pulled over the left shoulder, and pinned on the chest. The upper corners were tucked into the belt on either side, leaving a sort of pouch above the belt in back. In bad weather, this upper part could be wrapped around the shoulders and even drawn over the head if there was enough of it.
The Great Kilt was an extremely practical garment for the old highlanders: warm but not confining and almost waterproof due to the natural oils in the wool. If the highlander unexpectedly had to spend the night outside, he could even unbelt the kilt and roll up in it to sleep. With the suppression of the clan system and the industrial revolution, many highlanders were forced to find work in the industries of the Lowlands and England and there the great kilt was less practical. The kilt in its most common modern form (the Little Kilt) is supposed to have been devised by an English foundry owner who employed a number of Scots. A foundry is by nature a hot environment and the owner saw that his Scottish employees were often simply removing their kilts and working in their underclothes - if any. In the interests of safety and propriety, the owner suggested that the Scots pick out the seam between the two strips of cloth and wear only the lower one. The idea caught on and remains the most common form of the kilt today, although the Great Kilt is still seen occasionally. Nowadays, the usual kilt consists of 8 yards of worsted wool tartan cloth with the pleats stitched in permanently. It should hang to the knees and is fastened at the waist with buckles and at the bottom edge with a decorative kilt pin. The upper part of the Great Kilt is sometimes commemorated with the Plaid, a wide strip of tartan cloth pinned on the chest and hanging down the back to the knees or below. More recently, the there has developed a fashion for the Fly Plaid, a square yard of tartan or a longer, but narrower strip. The whole kit includes kilt hose - heavy stockings that come up to a wide cuff over the fullest part of the calf. Flashes (little strips of tartan cloth) hang below the cuff on the outside of the leg, and a Sgian dubh (pronounced skeen due - a small black handled knife) is tucked into the top of the right stocking. In place of pockets, a Sporran or belt pouch is worn in front below the belt; it is suspended by its own belt or in some cases decorative chains. Clothing for the upper part of the body depends on the occasion and may be anything from a casual shirt to a formal jacket and lace jabot. Casual headwear is a tam, or a beret tam with more shaping and a wider headband. For a more formal look there is the balmoral, similar to a beret tam but with a patch on the side to support a clan crest. The glengarry is sort of like the old military garrison cap and also has a place for a clan crest. All except the tam can be had with or without dicing - a checkerboard pattern of white and color on the headband. (Be warned: dicing is supposed to indicate loyalty to the Crown.)
For a definitive treatise on kilts and the Scottish costume in general, the Tartan Web site would be hard to match. It includes illustrated instructions for putting on the Great Kilt, a mass of information about tartans, cloth, and accessories, and pictures of a long list of tartans. TartanWeb also sells kilts and a complete line of accessories - the author can personally attest the fine quality of their kilts.  The Scottish Lion Import Shop also sells kilts, accessories and a large number of related items.  They have a tartan finder on the site and both their Scottish and Irish printed catalogs are good. They also have a great selection of headwear; the tams come in tartan, the others in solid colors.I have been very satisfied with anything I have bought from them. For the martial Celt (which may be redundant), Museum Replicas Ltd offers a Sgian Dubh, dirks, and authentic replicas of claymores and basket hilt swords. They also have period clothing and jewelry and sporrans. There is a woman in Albuquerque who has received the Scottish Qualifications Authority Award in kiltmaking. Her name is Kathy Lare and her phone and fax number is 505-890-6572. A couple of additional informative sites, although I haven't ordered from them: Linda Clifford has a lot of information as well as kilts and a wide selection of tartan fabric. The Clans and Tartans of Scotland has information on clans and tartans (obviously), maps, and other information on its page and available on CD.