There were three 84th regiments of foot. The first was disbanded in1765.
The second was the 84th of Foot(Royal Highland Imigrants) served in Canada during the American War of Independence.
The Third was the 84th(York & Lancaster)Regiment of Foot, amalgamated to become the 65th of Foot Yorks and Lancaster Regiment the old 84th becoming the second battalion.
I am a bit of a military history buff.
psusennes: