Late on the evening of Thursday 15 June 1815, in a long low room in the Rue de la Blanchisserie in Brussels that had until recently been a coachbuilder's showroom, the orchestra stopped playing and the guests drew back against the walls.

The doors opened, and in marched a party of kilted soldiers behind their pipers. They were sergeants and men of the 92nd Regiment, the Gordon Highlanders, in scarlet jackets and dark green tartan, and they had been lent for the evening by their colonel as a favour to the hostess. The Gordon name would later be associated with the elegant white-ground Gordon Dress tartan, a formal variation of the same family and regimental cloth. They laid their swords on the floor, and to the skirl of the pipes they danced: reels first, then the sword dance, stepping high and neat over the crossed blades.

The guests, who included the Duke of Wellington, the Prince of Orange, the Duke of Brunswick and most of the senior officers of the allied army, were delighted. One of the hostess's daughters remembered long afterwards that her mother had thought it would interest the foreigners to see them, and that some of the poor men who danced in the house that night were dead within days.

The Duchess's daughter

The hostess had a particular reason for asking for the Gordons. Charlotte, Duchess of Richmond, was herself a Gordon. Her father was the fourth Duke of Gordon, who had raised the regiment in 1794. Her mother was the celebrated Duchess Jane, who was said to have recruited for it at the country fairs of Aberdeenshire with the King's shilling held between her lips. The 92nd was, as far as she was concerned, the family regiment.

Her husband commanded the reserve in Brussels. The city that June was crammed with British officers and their families, and the Duchess had sent out some two hundred invitations to a ball. She had asked Wellington whether she should go ahead, with Napoleon known to be somewhere to the south. "Duchess," he is supposed to have said, "you may give your ball with the greatest safety, without fear of interruption."

He was wrong. That afternoon Napoleon had crossed the frontier with a hundred and twenty thousand men and was driving straight up the road to Brussels, aiming to split Wellington's army from its Prussian allies. The news reached Wellington in stages through the evening. He went to the ball anyway, to prevent a panic, and sat through supper talking cheerfully to his neighbour.

Then he asked his host whether he had a good map in the house. In the Duke of Richmond's dressing room, with the door shut, he looked at it and said: "Napoleon has humbugged me, by God. He has gained twenty-four hours' march on me."

Out in the ballroom, officers were being tapped on the shoulder and slipping away. Girls were in tears. The music went on. Byron, who was not there, made it the most famous party in English verse:

There was a sound of revelry by night, And Belgium's capital had gather'd then Her Beauty and her Chivalry...

By dawn the regiments were marching south. Some officers had no time to change, and fought the next day in silk stockings and dancing shoes.

Quatre Bras

The Gordon Highlanders reached the crossroads of Quatre Bras on the afternoon of 16 June, after a march of more than twenty miles in the heat, and were put straight into the line in a ditch beside the road, with Wellington himself sitting his horse just behind them. French cavalry charged them and were shot down. French infantry occupied a house and garden to their front, and the Duke turned to the regiment's commanding officer, Colonel John Cameron of Fassiefern, and said, "Now, Cameron, is your time. Take care of that road."

They took it with the bayonet. Cameron was shot from an upper window and mortally wounded. By nightfall the 92nd had lost nearly half its strength. Two days later, at Waterloo, what was left of them charged again.

Among the dead at Quatre Bras were men who had danced over the swords on Thursday night.

White for dancing

The tartan those sergeants wore was the Gordon: the dark Government sett of navy, green and black, with a single fine yellow line through the green. Its origin is as well documented as any. In 1793 a Huntly clothier named William Forsyth sent the Duke three samples, with one, two and three yellow stripes, and the Duke chose the first. The regiment wore it for two centuries, and the clan adopted it as its own.

The dress version is the same cloth let into the light. One of the broad dark bands is opened out into white, so that the navy, the green and the gold line stand on a clear white ground. The tartan archives date the oldest Dress Gordon to a collection of 1906, and note that it is based directly on the regimental pattern of the 92nd.

Dress tartans in general descend, it is thought, from the white plaids that Highland women once wore. In the twentieth century they found a second home. Highland dancing, the competitive art that grew out of exactly the kind of display the sergeants gave in Brussels, took up the white-ground tartans with enthusiasm, because they show up brilliantly on a platform and make a dancer's movements easy for the judges to read. At any Highland games in the world, the girls and boys dancing the Fling and the Sword Dance are very likely to be in Dress Gordon or something like it.

It is equally a tartan for formal evenings and for weddings, and Gordon Dress tartan is offered by retailers alongside the dark regimental sett and its weathered and ancient colourings, made to order as kilts, skirts, sashes and scarves, or sold by the yard.

And there is one garment in which a white-ground tartan looks finer than in any other.

Fergus, Ontario, a Saturday in August

James Gordon is thirty-one, a vet, and he is getting married at four o'clock in a stone barn outside Fergus, a town founded by Scots in 1833 that still holds one of the biggest Highland games in North America. His father, Alasdair, who is sixty-six and has worn the kilt all his life, is dressing him.

The kilt is on, eight yards of Dress Gordon. The Prince Charlie jacket and waistcoat are on. The sporran is hung. Alasdair opens a flat box and takes out something James has never worn before: a large square of the same tartan, fringed along all four edges, with one corner gathered into pleats.

"The fly plaid," says Alasdair. "This is the last thing. Stand still."

James knows, because his father has told him more than once, what the fly plaid is. The original Highland garment was the belted plaid, a single huge blanket of tartan, twelve or fifteen feet of it, which a man pleated on the ground, lay down on, and belted round his waist. The lower half made the kilt. The upper half was thrown over the shoulder, pinned, and could be pulled up over the head in the rain or wrapped round the body as a cloak. In the eighteenth century the lower half was cut off and tailored, and became the kilt we know. The upper half disappeared from daily life. The fly plaid is its ghost: a purely ornamental square, worn on the grandest occasions, hanging from the left shoulder down the back to about the knee.

They had bought it, with the brooch, from Scottish Kilt, who had made the kilt. Their fly plaids are described as being for formal Highland evening dress, pinned through the left epaulette over a Prince Charlie or a formal Argyll jacket, and matched to your kilt. That last phrase matters most. A plaid has to come from the same cloth as the kilt it is worn with, from the same maker and ideally the same run, or the two whites and the two greens will not agree, and in a white-ground tartan everyone will see it.

Alasdair works carefully. He passes the pleated corner up under the epaulette on the left shoulder of the jacket, from back to front, so that a few inches come through. He spreads the rest of the square so that it falls straight down James's back and does not pull. Then he takes the brooch, a round silver one with a thistle in the centre and an antique finish to match the buttons, and pushes its pin through the pleated corner and through the cloth of the jacket just below the shoulder seam, high on the chest. He steps back, looks, and shifts it half an inch.

"It wants to sit high. If it's too low it drags the jacket off your shoulder all night. And it goes on the left, always, so your sword arm's free."

"I don't have a sword, Dad."

"Your right arm's for your wife today. Same thing."

James turns to the mirror. From the front there is just the brooch, and a fold of white and green over his shoulder. When he turns, the plaid swings out behind him like a short cloak, the gold line catching the light. It changes the whole silhouette. He looks a foot taller.

"Take it off for the dancing," says his father. "It's for the ceremony and the photographs. Nobody can do a Strip the Willow in one. And when you take it off, give it to your mother, not the best man."

He straightens the brooch one last time. His hands, James notices, are not quite steady.

"Your great-great-grandfather was in the Gordons," Alasdair says. "He was at Loos. His grandfather was in them before him. There's a story in the family that one of ours danced for a duchess in Brussels the night before a battle, and I've never been able to prove it and I've never stopped believing it." He pats the brooch. "There. Now you're dressed."

The sound of revelry

The ballroom in the Rue de la Blanchisserie was pulled down long ago. The 92nd is gone too, merged and merged again into the Royal Regiment of Scotland. What survives of that night is a stanza of Byron, a handful of letters, and the picture they all share: kilted men dancing over crossed swords in a room full of candles, a few hours before marching out to die.

The white-ground Gordon is a tartan for the moments before and after, for the dance and the wedding and not the battle. With a plaid of it swinging from a man's left shoulder, pinned by his father's hands, it is about the most splendid thing Highland dress can do.