The hackle is a feather plume (most plumes are made of horsehair) that is attached to the headdress.
In the British Army and the armies of some Commonwealth countries the hackle is worn by some infantry regiments, especially those designated Fusilier regiments (such as The RRF and The RHF), and those with Scottish and Northern Irish origins. It was commonly attached to the feather bonnet worn by Highland regiments (now usually only worn by drummers, pipers and bandsmen). The colour of the hackle varies from regiment to regiment, notably:
- Red - Black Watch
- White - Royal Welch Fusiliers and Royal Highland Fusiliers
- Blue - Queen's Own Highlanders
- Green - Royal Irish Regiment
- Yellow - Lancashire Fusiliers
- Red/White - Royal Regiment of Fusiliers
- Maroon/Light Blue - A (Liverpool Irish) Troop, 208 (3rd West Lancashire) Battery RA(V)
- Blue/Red - Royal Warwickshire Fusiliers
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