Besides the Cameronians (The Scottish Rifles), the following regiments wear the Douglas tartan:

7th Gurkha Rifles (amalgamated)
The Royal Gurkha Rifles
The Perth Regiment (Canada)
The Witwatersrand Rifles (South Africa)

The majority of these regiments wear the Douglas in trews, although pipers & drummers wear kilts. Rifle Regiments (The Gurkhas are considered Rifles) traditionally wear "rifle green", so the Douglas is a perfect tartan for them. The Wits in South Africa are still around as a militia unit, not sure about the Perth Regiment.

There is a book about the 2nd Battalion Cameronians (The Scottish Rifles) entitled "Morale" by John Laffin which deals with their service in WWI, and it goes briefly into a history of the Regiment.

1st Batt., Cameronians, were raised from the conservative Presbyterian faction known as Cameronians (named after their martyred leader, Richard Cameron), and was the only regiment raised from a religious group in the British Army. The Cameronians would post sentries at the four corners of the church on Sunday morning, and an officer would shout "All Clear!" before the Padre began the service, in reference to the secret Presbyterian meetings on hillsides and in fields called conventicles where armed covenanters protected the flock from government soldiers.

2nd Batt., Cameronians, the Scottish Rifles, had no such religious overtones & referred to themselves as the Scottish Rifles to distinguish themselves from 1st Batt. They were raised in the 1790's during the wars with Revolutionary France.

In 1968, when faced with amalgamation with another regiment, the Cameronians chose to disband, although they are still around as a Territorial Army Battalion.

Cheers,

T.