I have a nice vintage collection of what are known as “Real Photo” postcards. Families or traveling photographers took photographs that could be printed on the front side of a postcard that could be mailed to relatives and friends. See the Wikipedia description of the Real photo postcard here:
Although not mass-produced as were the souvenir or commercial post cards, the individual images were likely to be printed on many postcards. This is the reason one might find the same print in many photo albums for the same (and extended) family.
The following photo of my great-grandparents was taken between 1905 and 1910. There is no writing on the back of this postcard.
The next postcard provided me with clues while researching a complicated family line. It includes a photo of a female relative with her children and second husband. On the back of the photo postcard, she wrote to her sister and mentioned the recent funeral of her ex-husband’s father. Note the damage to the written side of the postcard. The postcard was removed from an album in which the photos had been glued to the black album pages.
I have a few mass-produced antique postcards in my collection but I have a preference for the Real Photo postcards because they contain the only known images of some of my family.