To start with, tartan itself is very historical in Scotland. The oldest piece of tartan found in Scotland comes from Falkirk and dates from the 3rd century CE/AD. It’s a simple pattern made with undyed light and dark wool. There are older tartan finds from across Europe, and also China. Many of the European finds are from sites associated with Celtic cultures.

However, clan tartans as we know them today are a relatively recent invention. There is some evidence for the idea that certain setts (patterns) may have been more popular in certain geographic areas, and locally available plants may have affected the colour choices (at least among those who couldn’t afford imported dyes.) There’s no evidence that people were wearing specific clan identifying tartans, though. Early modern portraits suggest that people might wear two or three tartans in one outfit!

After the Jacobite rising in 1745/46, there was a proscription on “Highland dress.” This was not a banning of tartan, but of men wearing kilts. There was also a ban on weapons. These proscriptions lasted 45 years and applied to all Gaels, whether they had fought on the Jacobite side, the Hanoverian side, or stayed neutral. There are multiple Gaelic poems from this era complaining about wearing trousers and how unpleasant they are.

The only place Gaels were allowed to carry weapons and wear the kilt was in the British army. This is really where the idea of an identifying tartan starts, since the regiments would bulk buy rolls of the same tartan for uniforms.

The history of what was going on here is complex, but basically Anglophone sources paint the Gaels as wild, savage and threatening. Slowly that changes into still wild, still savage, but romantic, simpler, noble. The Lowland Scottish writer Walter Scott was an enthusiastic proponent of this romantic warrior image. He also got put in charge of court spectacle when King George IV visited Edinburgh in 1822. He was the first Hanoverian king to visit Scotland and the first reigning monarch to visit Scotland since 1651. So it was a big deal. Scott’s idea was to revive Highland dress and have each noble appear with their retinue in full kilt and regalia. The king was also outfitted in a kilt in the newly created Royal tartan (later the Royal Stuart tartan), which he wore with pink hose. This was intended to emphasize his connection to the Stuart line - one of his great great great grandmothers was a Stuart and thus he was descended from the Scottish kings.

So the Scottish nobles scrambled to get outfitted in kilts (some borrowed them from the Highland regiments.) This event started a trend for tartan and kilts as formal wear. It’s also in this time period that the idea of ancient clan tartans begins to form.

In 1829 two enterprising English brothers show up and begin to tell people that they are secretly the grandsons of Charles Edward Stuart and that they have a 15th century manuscript detailing the clan tartans pre-1745. This is all a complete fraud, but in 1842 the older brother, John Allan, publishes a book using the name John Sobieski Stuart. (Maria Sobieska was Charles Edward Stuart’s mother.) This book, the Vestiarium Scoticum, was purported to be that manuscript and featured full colour illustrations of the tartans. I’ve seen a copy of it - it’s gorgeous. There are tartans listed for Highland clans and Lowland families. These seem to have been invented out of whole cloth, as it were. There was considerable scepticism at the time about all of the Sobieski Stuarts claims - Walter Scott himself dismissed the idea that there were ever clan tartans in a letter to a friend - but there were a lot of people excited by the idea. Secret princes with hidden ancient documents offering fancy dress that allowed people to proclaim their affiliations? It appealed.

The Sobieski Stuarts got a lot of social capital and money out of their deceptions, hobnobbing in circles where the sons of a former sea captain who spent a lot of time evading his creditors by traveling around Europe might not have been welcomed. Tartan manufacturers, weavers and tailors got a guide book they could use to help people decide which tartan to buy.