I remember Polaroid black & white print cartridges that supplied self-adhesive postcards as backings for the prints. After you stuck the postcard to the print you applied the hypo, that was on a padded handle also supplied in the package, to the photo. You of course had the option of putting a stamp on it, adding a message, and mailing it, but I don't remember ever doing it.

My in-laws (from both of their respective families) have a bunch of really great European family "postcard" pictures, the earliest dating from maybe first decade of the 20th Century, the last just predating the Second World War.

These are the only old photos they have multiple copies of, which leads me to think that it was an economical way of printing multiple copies so that everyone got a copy (and maybe you'd mail a copy to distant relatives). Being printed on heavy card stock rather than glossy photographic paper probably kept the costs down, the same as now.

And the added bonus is that multiple copies means there is more chance of the images surviving, which is good for descendants and history buffs.

---lothlaurien

In those days -- even up to the Second World War, it was usual to have photos of yourself printed as postcards. I have a few of my great-grandparents. Postcards have, as the years have gone by, changed to become tourist tool to an obsolete way to communicate.