A tartan is a specific woven pattern that often signifies a particular Scottish clan in the modern era. The pattern is made with alternating bands of coloured (pre-dyed) threads woven as both warp and weft at right angles to each other. The weft is woven two over - two under the warp, advancing one thread each pass, forming diagonal lines. The resulting blocks of colour repeat vertically and horizontally in a distinctive pattern of squares and lines known as a sett. Kilts almost always have tartans. Tartan is also known as plaid in North America, but in Scotland this word means a tartan cloth slung over the shoulder or a blanket.
Textile analysis of fabric from Indo-European Tocharian graves in Western China has shown similarities to the Iron Age civilizations of Europe dating from 800 BC, including woven twill and tartan patterns strikingly similar to Celtic tartans from Northwest Europe. The Celts wore coats set with a pattern of checks close together and of varied colors, similar in fashion to the Scottish, Irish, and Welsh tartans. Tartan patterns have been used in Scottish, Irish, Northumbrian (north east English) and Welsh weaving for centuries. A possible predecessor dating from the 3rd century, found near the Antonine Wall and known as the "Falkirk sett", has a checked pattern in two colours identified as the undyed brown and white of the native Soay sheep. The fabric had been used as a stopper in an earthenware pot containing a hoard of silver coins.
Particoloured cloth was used by the Celts from the earliest time, but the variety of colours in the breacan was greater or less, according to the rank of the wearer. That of the ancient kings had seven colours, that of the druids six, and that of the nobles four. In the days of Martin Martin, the tartans seemed to be used to distinguish the inhabitants of different districts and not the inhabitants of different families as at present. He expressly says that the inhabitants of various islands were not all dressed alike, but that the setts and colours of the various tartans varied from isle to isle. As he does not mention the use of a special pattern by each family, it would appear that such a distinction is a modern one, and taken from the ancient custom of a tartan for each district, the family or clan in each district originally the most numerous in each part, eventually adopting as their distinctive clan tartan, the tartan of such district. Martin’s information was not obtained on hearsay: he was born in Skye, and reared in the midst of Highland customs.
For many centuries, the patterns were loosely associated with the weavers of a particular area, though it was common for highlanders to wear a number of different tartans at the same time. A 1587 charter granted to Hector Maclean of Duart requires feu duty on land paid as 60 ells of cloth of white, black and green colours. A witness of the 1689 Battle of Killiecrankie describes "McDonnell's men in their triple stripes". From 1725 the government force of the Highland Independent Companies introduced a standardised tartan chosen to avoid association with any particular clan, and this was formalised when they became the Black Watch regiment in 1739.
The most effective fighters for Jacobitism were the supporting Scottish clans, leading to an association of tartans with the Jacobite cause. Efforts to pacify the Highlands led to the 1746 Dress Act banning tartans with exemptions for the military and the gentry. Soon after the Act was repealed in 1782 Highland Societies of landowners were promoting "the general use of the ancient Highland dress". William Wilson & Sons of Bannockburn became the foremost weaving manufacturer around 1770 as suppliers of tartan to the military. Wilson corresponded with his agents in the highlands to get information and samples of cloth from the clan districts to enable him to reproduce "perfectly genuine patterns" and recorded over 200 setts by 1822, many of which were tentatively named. The Cockburn Collection of named samples made by Wilsons was put together between 1810 and 1820 and is now in the Mitchell Library in Glasgow. At this time many setts were simply numbered, or given fanciful names such as the "Robin Hood" tartan.
By the 19th century the Highland romantic revival inspired by James Macpherson's Ossian poems and the writings of Walter Scott led to wider interest, with clubs like the Celtic Society of Edinburgh welcoming Lowlanders. The pageantry invented for the 1822 visit of King George IV to Scotland brought a sudden demand for tartan cloth and made it the national dress of the whole of Scotland, with the invention of many new clan tartans to suit.
The naming and registration of official clan tartans began on April 8, 1815 when the Highland Society of London (founded 1778) resolved that all the clan chiefs each "be respectfully solicited to furnish the Society with as Much of the Tartan of his Lordship's Clan as will serve to Show the Pattern and to Authenticate the Same by Attaching Thereunto a Card bearing the Impression of his Lordship's Arms." Many had no idea of what their tartan might be, but were keen to comply and to provide authentic signed and sealed samples. Lord Macdonald was so far removed from his Highland heritage that he wrote the Society: "Being really ignorant of what is exactly The Macdonald Tartan, I request you will have the goodness to exert every Means in your power to Obtain a perfectly genuine Pattern, Such as Will Warrant me in Authenticating it with my Arms."
In general a tartan is a plaid which belongs to someone, some thing, some place, and/or some family. For examples of these various kinds, which now number over 5,000 visit 
The tartan of a Scottish clan is a sequence of colors and shades unique to the material, authorised by the clan society for use by members of that clan for kilts, ties, and other garments and decorations. Every clan with a society has at least one distinct tartan. While "heraldic" in the sense of being visual representation of blood relation, they are not "Scottish heraldry", strictly speaking. In Scotland, heraldry is protected under the law by the court of the Lord Lyon, King of Arms, and there are penalties for bearing an unauthorised Coat of arms. Any tartan specified in a Grant of Arms by the Lord Lyon is registered by him, but there is no legal prohibition against wearing the "wrong" tartan. It is considered proper to wear a clan tartan if the wearer is associated with the clan by name, by blood or by legal adoption. It is also proper to wear a tartan ascribed to the district, county, or shire.
In the border areas of England abutting Scotland, tartans are called 'checks'.
The Irish people had clans too, except each clan mostly lived within its own community, also known as a county. So far, there are 32 counties in the Republic of Ireland and Northern Ireland, and they each have at least two very distinctive tartans. So far, there are just a little under 70 Irish tartans, ranging from county and crest tartans to the popular Irish National.
Other modern tartans
In addition to the clan tartans, there are many tartans regsitered for families, districts, institutions and even specific commemorative "memorials" for events or persons. Further, tradition reserves some patterns for use by Scottish Highland military units of the United Kingdom and Commonwealth countries.
Those units associated with the British Royal Family use the Royal Stewart Tartan regardless of whether they are affiliated by blood to the Stewart clan. This is because of the Royal Family's Stewart ancestry through James VI of Scotland. However tartan is pretty inclusive. There are tartans for military forces like the Royal Air Force & Royal Canadian Air Force, commercial companies, special interest groups like Amnesty International, religious movements (including Hare Krishna), cities, football clubs, dancing and whisky-drinking societies, non-British Celtic groups such as French Bretons and Spanish Galicians, commemorations and regions of the world where people of the Scottish Diaspora live. As a result most people, whether of Scottish ancestry or not, can find some tartan which is significant for them. There are also general fashion tartans, not officially registered in Scotland, for those who do not care about the significance.
British Airways used a tartan design as part of its ethnic tailfin rebranding. This design, Benyhone or "Mountain of the birds," was one of the most widely used designs, being applied to 27 aircraft of the BA fleet.
Queen of Scots the US importer of Edinburgh Liqueur and The California Pure Malt and Indiana Pure Malt whiskies use a tartan developed exclusively for its western hemisphere marketing of its products. The Queen of Scots tartan may be seen at Bonbright Woolens on their web sites. The tartan was designed by David McGill of Edinburgh, the interantionally noted tartan designer.
Commemorative of The Da Vinci Code, and highlighting awareness of the Jesus and Mary Magdalene enigma encoded in stone at Rosslyn, a commemorative Tartan - the 'Rosslyn Da Vinci' – was created in spring 2006 by a group of Historians local to Rosslyn. The Rosslyn Chapel Tartan is registered to David McGill of Edinburgh at International Tartans.
The Clergy are the only profession represented by a separate tartan. The legend that goes along with this is that they needed a separate tartan to wear instead of their own family's so that they would not be attacked by members of their new congregations who were feuding with their clan.
In the Celtic regions of Cornwall and Wales tartans and kilts have been adopted as part of the 19th and 20th century Celtic revival and the traditional Northumbrian tartan, known in Scotland as the Shephard's Tartan, perhaps the oldest tartan design in Britain, is common and worn by Northumbrian Pipers in the north of England.
The word 'Tartan' is also used as a prefix to denote something of Scottish origin, for example the term 'Tartan Army' is used to refer to fans of the Scottish national Football (soccer) team. The Rev Donald Caskie, a Church of Scotland minister, became known as the Tartan Pimpernel for helping Allied service personnel to escape from occupied France during World War II.
Other than those tartans specifically registered to Clan Chiefs, there is no official tartan registry. The closest thing to a formal registry is The World Registry of Tartans (see link below).
A bill before the Scottish Parliament to establish a formal registry of tartan under the aegis of The Lord Lyon has been languishing since 2001 when a petition to the Scottish Parliament was sent appealing to the Scottish Parliament to do so. This bill is seemingly being held up by the commercial interests of some of the tartan mills in Scotland. The reasons for needing aformal registry are severalfold: there are no clear definitions of colours, there is no standard definition of the sett, i.e., geometry, or spacing of the tartan's patterns. This lack of definitions has led to dumping of miscolored and malshaped tartans in the North American markets.
- Tartan Day, a day set aside for the celebration of the Scottish influence on North America, Australia and New Zealand, but ironically not much celebrated in Scotland itself. It is the anniversary of the Declaration of Arbroath.
- Tartan techno, a style of techno music from Scotland.
- Vestiarium Scoticum, a source of many "original" clan tartan patterns
- The World Registry of Tartans.
- The Official State Tartan of California.
- Scans of Tartan Cloth of over 450 different tartans.
- Tartans Authority The official Scottish tartans industry registrar.
- Fabric and Merchandise in Over 500 tartans
- The Scottish Tartans World Register. Over 3000 tartans with images.
- The Scottish Tartans Society
- A Tartan for New York City out of respect for the September 11, 2001 attacks.
- Searchable directory of all known production tartans
- Tartans - Scotland.com
- Cornish Tartans
- Welsh Tartan
- Photoshop plugin that can create Tartans
- Clan and Tartan Search Page
The Carnegie Mellon University student newspaper is also called The Tartan. In addition, the Carnegie-Mellon tartan is considered to be an official school color. The Carnegie Mellon sports teams are nicknamed the Tartans and their mascot is "Scotty". 
- Tartans, ed. Blair Urquhart, The Apple Press, London, 1994, ISBN 1-85076-499-9
- Clans and Tartans—Collins Pocket Reference, George Way of Plean and Romilly Squire, Harper Collins, Glasgow 1995, ISBN 0-00-470810-5
- "The Invention of Tradition: The Highland Tradition of Scotland", Hugh Trevor-Roper, in The Invention of Tradition, ed. Eric Hobsbawm and Terence Ranger, 1983, ISBN 0-521-24645-8.
- History of highland dress: A definitive study of the history of Scottish costume and tartan, both civil and military, including weapons, John Telfer Dunbar, ISBN 0-7134-1894-X.