Within Departments (Colonial Office, War Office etc.) records are grouped archivally into classes, and assigned a number. eg. CO 201 NSW Original Correspondence.
A class is a group of records located together on the basis of form (eg. WO 22 Pension Returns), or geographical emphasis (eg. CO 13 South Australia Original Correspondence).
See Accessing the collection for a guide on searching by class number.
AJCP Handbook No.2 – Colonial Office (Class Numbers)
The Class Number is followed by a Piece Number.
eg. CO 201/ pieces 1-107
The piece number is the section of the microfilm reel on which the record can be found, and is a running number.
AJCP Handbook No.1 – A shelf list (inventory) of all 10,000 reels in the AJCP (Class and Piece numbers)
In the next example, (AJCP Handbook No 4 – War Office), records for Class WO 12 Muster Books and Pay Lists of British Regiments are displayed.
Each record has four elements: Reel No. (PRO 3698), Piece No. (2216), Date (1834 Apr – 1835 Mar), Description of the actual record: 4TH Regiment Muster and Pay Lists.
As you scroll through reel PRO 3698 you will come to a tab marked 2216 (piece no.), the date the piece covers – Apr 1834 – Mar 1835, and a description of the record – 4th Regiment Muster and Pay Lists.
AJCP Handbook No.4 War Office
See Accessing the collection for a guide on searching by piece number.