Ordnance Survey maps are available for different time periods, known as epochs. Of particular interest here are the first four epochs which equate to the first edition and subsequent revisions of the County Series as follows:
First edition County Series published between 1843 and 1893 = epoch 1
First revision County Series published between 1891 and 1912 = epoch 2
Second revision County Series published between 1904 and 1939 = epoch 3
Third revision County Series published between 1919 and 1943 = epoch 4
Extract from OS epoch 4 sheet
The County Series maps were produced at scales of 6" to the mile and 25" to the mile and provide a great deal of detail for historical research.
These maps were produced on a grid for each county. Each 6" map sheet is usually given a Roman numeral to identify it within the grid, e.g. XXX refers to the 6" sheet number 30.
Some 6" maps are available as one map sheet per each grid square, whilst others are more readily available split into quarters with each of the quarter sheets designated NW, NE, SW and SE. Some editions may also be available as double quarter sheets, e.g. NW and NE quarters printed together on one sheet.
Each 6" grid square is subdivided into 16 equal areas each given an Arabic number; each of these 16 areas is equivalent to the coverage of an individual 25" map sheet, e.g. XXX/3 refers to a 25" map sheet which covers part of the area covered by the XXX 6" sheet.
A researcher using the Ordnance Survey maps needs to become familiar with the different signs and symbols used on the maps to denote features and land use types, particularly as some of these have changed with the different epochs. Printed guides explaining the symbols are usually available at County Record Offices. You can download an example below.