If you work with Japanese picture postcards as historical source material, you have probably noticed that most surviving cards lack postmarks and other temporal indicators.
The back is divided so that 1/2 of the space is reserved for a message, and 1/2 is reserved for the address.
Based on research into the postmarks and cards themselves, contemporary journalism, secondary scholarship, and some postcard websites, the EAIC has devised the following periodization scheme: Period I.
Undivided back: October 1, 1900-March 27th, 1907 Period II.
1/3 divided back: March 28, 1907-February 28, 1918 Period III.
March 1st 1918-February 14, 1933: 1/2 divided back w/ 郵便はき: February 15, 1933-August 15, 1945 On October 1st, 1900, it became legal in Japan for private companies and individuals to produce picture postcards that the Japanese postal authorities would accept and deliver at the postcard rate.