An article on "Union Jack" in the NY Times:
When Balanchine Sent in the Clans
By TOBI TOBIAS
In a New York City Ballet studio, backed by a tape of implacable drumbeats, a lesson begins, emphasizing driving feet, lashing kicks set against a twisting torso and sudden, ominous halts, all executed with ferocious force at a relentless pace.
"The movement takes more than just physical energy," Karin von Aroldingen, a ballet mistress with the company, explains. "You have to give it will power — and spirit."
The lesson is how to be the chief of a vehement 10-woman marching clan, a role that Ms. von Aroldingen created in 1976 for George Balanchine's "Union Jack."
For more, clik here
An interesting look at "Union Jack", though I think the history is a bit blurred. There is no doubt that Balanchine's experiences at the Edinburgh Festival heavily influenced the first part of Union Jack. Anyone who has seen the Edinburgh (Military) Tattoo could not help but to notice the similarities.
To clarify, each 'clan' (there is no such thing as an all male or all female clan - a biological impossibility - clans are linked by a common family name) has a distinct tartan or tartans. Some clans have just a single tartan, others have a number of tartans - plain, dress, hunting etc. The names of the groups in 'Union Jack' reflect the tartan worn by the dancers and thus the name of the clan represented.
At the real Edinburgh Tattoo, the tartans are most notable in the massed drums and pipes (usually 100+ of each) where most of the pipers & drummers from each army/navy/marine/airforce regiment wear the regimental tartan and regalia. Some regiments, often foreign, do not wear kilts, but the majority do and it's a pretty spectacular sight to watch from one's window each night (though by the third week it gets a bit wearying).