Part III : Captivity #Section IV : Relations of prisoners of war with the exterior - Art. 36.
Art. 36. Each of the belligerents shall fix periodically the number of letters and postcards which prisoners of war of different categories shall be permitted to send per month, and shall notify that number to the other belligerent. These letters and cards shall be sent by post by the shortest route. They may not be delayed or withheld for disciplinary motives.
Not later than one week after his arrival in camp, and similarly in case of sickness, each prisoner shall be enabled to send a postcard to his family informing them of his capture and the state of his health. The said postcards shall be forwarded as quickly as possible and shall not be delayed in any manner.
As a general rule, the correspondence of prisoners shall be written in their native language. Belligerents may authorize correspondence in other languages.