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 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.

British Army

In the British Army, the colours of the three fusilier regiments are as follows:

• Royal Highland Fusiliers: White
• Royal Regiment of Fusiliers: Red over white
• Royal Welch Fusiliers: White

Non-fusilier regiments which wear the hackle are:

• Argyll and Sutherland Highlanders (feather bonnet only): White
• Black Watch: Red
• The Highlanders (Seaforth, Gordons and Camerons): Royal blue
• Irish Guards (pipers on caubeen only): St Patrick's blue
• Liverpool Scottish (now a platoon of A (King's) Company, King's and Cheshire Regiment ): Royal blue
• London Irish Rifles (now D (London Irish Rifles) Company, London Regiment): St Patrick's blue
• Royal Irish Regiment (as the direct descendent of two regiments of fusiliers): Green
• Royal Scots Dragoon Guards (pipers on feather bonnet only): White
• Scots Guards (pipers on feather bonnet only): Blue over red

There were several other fusilier regiments which have been amalgamated and no longer exist. Their colours were as follows:

• Lancashire Fusiliers: Primrose yellow
• Royal Fusiliers (City of London Regiment): White
• Royal Inniskilling Fusiliers : Grey
• Royal Irish Fusiliers : Green
• Royal Northumberland Fusiliers : Red over white
• Royal Scots Fusiliers: White
• Royal Warwickshire Fusiliers : Blue over red

Former non-fusilier regiments, now amalgamated, which also wore the hackle were:

• Gordon Highlanders (feather bonnet only): White
• Queen's Own Cameron Highlanders : Royal blue
• Queen's Own Highlanders : Royal blue
• Queen's Royal Irish Hussars (pipers on caubeen only): White over red
• Royal Irish Rangers: Green
• Royal Corps of Transport (pipers on feather bonnet only): Red over white over blue
• Royal Ulster Rifles : Black
• Seaforth Highlanders (feather bonnet only): White

The fictional regiment featured in the series Soldier Soldier is also a fusilier regiment:

• King's Own Fusiliers: Blue over white

Canadian Army

There are also several fusilier regiments in the Canadian Army which wear the hackle (the French-speaking fusilier regiments do not appear to do so):

• Princess Louise Fusiliers : French grey
• Royal Highland Fusiliers of Canada : White

Scottish-influenced non-fusilier regiments which wear the hackle include:

• Argyll and Sutherland Highlanders of Canada (drummers on feather bonnet only): White
• Black Watch of Canada : Red
• Calgary Highlanders (drummers on feather bonnet only): White
• Cameron Highlanders of Ottawa : Royal blue
• Canadian Scottish Regiment (drummers on feather bonnet only): White
• Essex and Kent Scottish Regiment (feather bonnet only): White
• Lorne Scots : Primrose yellow
• Seaforth Highlanders of Canada (drummers on feather bonnet only): White

South African Army

Scottish- and Irish-influenced regiments which wear the hackle include:

• South African Irish Regiment : Green
• Transvaal Scottish Regiment: Red
• Witwatersrand Rifles: Black