Piece number (Contains between 100-400 pages)
A PRO reference number referring to a number of Enumeration Districts represented in this case by a file R12/XXXX.
Enumeration District (Contains about 50 pages)
This refers to a group of streets and has a special sheet listing which streets are in the following pages.
Folio (Reference to two pages)
The original book pages were stamped with a folio number on every other page in the top right hand corner.
These are provided by the PRO for areas having a population of more than 40,000.