First, remember that the Glengarry is a relatively recent development, and is essentially a blocked Balmoral bonnet. Glengarries were adopted by the Regular Scottish regiments beginning in the 1840s.
As best I can recall dicing begins appearing on Scots bonnets in the mid-18th century.
Initially the dicing was a single row. By the late 18th century three-row dicing had become standard in the Scottish regiments.
How it originated is perhaps open to some speculation. I've often heard the theory that a ribbon began to be set into the bottom edge of the bonnet, laced in and out of a number of slots, and tied at the back, in order to adjust the size of the bonnet.
The Black Watch and The Cameron Highlanders have long worn plain Glengarries, that is, without dicing.
As to why pipers of all regiments wear plain Glengarries, remember that the typical military piper's costume was devised in the 1840s for the pipers of The Cameron Highlanders, and later adopted by the other regiments. Certain Cameron-specific features of the costume (the plain Glengarries and the green doublets) were adopted without change by the other regiments. (One would have expected diced Glengarries and yellow doublets with the pipers of The Gordon Highlanders, for example.)
Time for pretty pictures!
Mid-18th century painting showing a red band around the bottom of the traditional blue Scots bonnet

Fully developed three-row dicing by the end of the 18th century

An early Glengarry worn with civilian costume prior to Glengarries being adopted by the army

Bonnet with one-row dicing in the 1860s, by which time it seems to have been rare

Two Sergeants of the 78th Highlanders pre-1881, plain Glen for the piper and diced Glen for the non-piper soldier
Last edited by OC Richard; 10th April 18 at 06:45 PM.
Proud Mountaineer from the Highlands of West Virginia; son of the Revolution and Civil War; first Europeans on the Guyandotte
Bookmarks