Letters and Postage Rates
There was a time in England when postage rates were very high. Only rates for newspapers were low. People who had little money but much time sent letters to their friends with newspapers. They made very small pin-pricks under letters and wrote large letters in this way.
Soldiers often used this kind of letter, too, to tell their families where they were. A soldier sent home a map with a pinprick at the proper place. Their mothers or wives held up the maps to the light and saw where the pin-prick was. The system was not very successful because the censors understood everything very quickly. They let the maps go through but they made hundreds of pin-pricks in them first.