1. United States:
   1. "POST CARD" allowed on back after December 24, 1901
   2. up to 1907 no messages were allowed on back, only address
   3. published Post Cards divided backs after March 1, 1907
2. Divided backs
   1. Great Britain, January 1902
   2. Germany, 1905
   3. France, 1904?
3. Use stamps as a ball park figure for age of a Post Card
4. Written date by sender
5. Modifications to ship (hard to spot sometimes)
6. Port ship is in (i.e. a ship may have only visited a port once)
7. Text on front or back
8. Post Card back information
9. From May 2, 1916 to approximately December 1918, British Ships were not allowed to be shown on Post Cards due to British censorship
10. On June 3, 1918 the Inland postal rate was raised to 1d in Great Britian
11. Flag the ship is flying (i.e. the ship could be a captured or resold ship)
12. The history of the ship (launch, completion, sinking, break up, etc.)
13. Different colour divided back line means a line was added later to an old card or not parallel to side of card
14. "This space used for communication", 1904+
15. "This space for communication", 1907+
16. publisher

An excellent source for Post Card back dates is the book - Prairie Fires and Paper Moons: The American Photographic Postcard:1900-1920 by Hal Morgan and Andreas Brown. Published by David R. Godine, Boston: 1981. This book focuses on the American postcards. The Appendix describes the methods for dating photographic postcards.