This was partly in response to the Tithe Commutation Act 1836 which led to calls for a large-scale survey of England and Wales.
In later editions the six-inch sheets were published in "quarters" (NW, NE, SW, SE), each covering an area of three by two miles on the ground.
[2][3] From the late 19th century to the early 1940s, the OS produced many "restricted" versions of the County Series maps and other War Department sheets for War Office purposes, in a variety of large scales that included details of military significance such as dockyards, naval installations, fortifications and military camps.
Apart from a brief period during the disarmament talks of the 1930s, these areas were left blank or incomplete on standard maps.
The de-classified sheets have now been deposited in some of the Copyright Libraries, helping to complete the map-picture of pre-Second World War Britain.