Hi Jennie
Presumably you are referring to the Great War period. Regular battalions normally consisted of a 1st and 2nd battalion, with a 3rd Reserve battalion, usually at a home depot.
There were also Territorial battalions affiliated to their Regular regiments, these Territorials served at home. On the outbreak of war and the subsequent actions, it became quite clear that the Territorial battalions in the field would need trained reinforcements. Therefore these battalions began to recruit men as reserves for the Territorial battalions in the field.
A example might be, (but there were variances by regiments.) The 1/5 would be the battalion in the field, then the 2/5 would be the first to reinforce the 1/5: Then the 3/5 would be the youngest/latest recruits to go through training.
Thus 2/5th Battalion Territorial Battalion:
Formed at Grimsby in February 1915. July 1915 : attached to 177th Brigade, 59th Division. 8 May 1918 : reduced to cadre. 29 May 1918 : transferred to 21st Brigade, 30th Division. 28 June 1918 : transferred to 66th Division. 31 July 1918 : absorbed by 1/5th Bn. see link here:
http://www.rootschat.com/links/01t7/Not easy to follow is it, as every regiment seemed to have its own idea on numbering battalions. Generally speaking, it was done on counties and localities where the men's regiments were raised.
