A true symbol of vacations, postcards are also very useful for genealogy - we explain!
©galargues.fr
You have surely already found them in your family archives, vintage postcards are, just like your family photos, a reflection of an era. With illustrations of photos of your ancestors' living places and their handwritten life stories, vintage postcards are precious little treasures to keep carefully.
But first, what is the origin of postcards?
1869: The first postcard appears in Austria. The French, however, fear a lack of discretion as these cards travel without an envelope.
Source: Postcard Museum
1870: During the Franco-Prussian War, the Society for Aid to Wounded Soldiers of the Army and Navy uses a stock of pre-paid cards for the wounded and besieged to communicate with their families.
Source: Memory of History
1872: The unillustrated postcard comes into effect thanks to the Finance Law at a rate of 10c for a local card (yellow - intended for France and Algeria with delivery in the same city or same office) and 15c for those that will travel further. At the same time, the law allows "open correspondence." It takes until January 15, 1873 for the postcard to be truly used. At that time, it measures 12 cm x 8 cm and weighs between 2g and 5g.
Source: multicollection.fr
1873: The stores A la Belle Jardinière create their own postcards with small black and white illustrations representing their buildings on Rue du Pont-Neuf.
1875: The private industry is now allowed to manufacture postcards.
1881: Creation of the first illustrated postcard by the Post Office Savings Bank.
1889: The recipient's address occupies the entire front - the image is cropped to leave some space for writing. The same year, for the World's Fair, a drawn card representing the Eiffel Tower sells 300,000 copies.
Source: Philapostel Bretagne
1891: Dominique Piazza, a Marseille photographer, is one of the first to market a postcard with a photographic image reproduction. These photo postcards will be rare until 1897 as manufacturing costs and technical processes are high. At the end of the century, the printer Neurdein will publish postcards for each major city in France. We also find at this time the "Gruss," German cards that address wishes and thanks.
Source: Philapostel Bretagne
1903: The Post Office evolves the standard - the entire front will be dedicated to the image and the back will be for writing and the recipient's address.
1900-1914: This is the golden age of the postcard, with 800 million copies printed. It's also the time when it adopts color and photochromy. Postcard publishers develop rapidly.
1914-1918: Military authorities encourage their use to convey patriotic and propaganda messages.
1920: This is the beginning of the decline of the postcard. Publishers produce lower quality postcards for profitability, and new communication methods replace it in daily life.
🌟 Good to know: We refer to CPA - old postcards - for all cards dating before 1940, CSPM - semi-modern postcards between the 1930s and 1970, and modern postcards for the most recent ones.
As we said in the introduction, postcards, through their illustrations and sent messages, reflect an era and the history of the country. Therefore, they are genuine sources for your genealogy: to put images on a place, to learn more about the life of your ancestors, and to discover interesting correspondences. 
There are many ways to find old postcards (CPA), in addition to searching through your family archives or rummaging through boxes at flea markets, several online sites can help:
- cartes-postales-anciennes.com : a site by the CDIP
- Cartorum : a collaborative project for cataloging and locating old postcards 
- CPArama : the forum for old postcard collectors
- Geneanet : a genealogy site's postcard database
- PostcardTree : foreign postcards (mostly American)
To find old postcards for a specific location, try searching directly on search engines. There are several sites dedicated to regional and local views.
There are also specialized commercial sites for selling old postcards like Delcampe, Bartko Reher CPA, Fortunapost, Collection-jfm... It is also possible to find old postcards on second-hand sales sites like Leboncoin or Ebay.
To avoid any deterioration of your postcards (detaching of pages, mold or dust spots, damaged corners...), enthusiasts of old documents agree that it is important to keep old postcards (just like any other archives) away from humid areas, light, and dust.
It is therefore recommended to store them in a dry place, in a neutral cardboard box with a doubled bottom, a case or an acid-free plastic sleeve certified PAT and with neutral pH.
It is entirely possible to start a genealogical search from an old postcard. In fact, the more information the card contains, the easier your search will be.
You can scrutinize the illustration for the slightest clue: store names, street numbers, year, etc.
You can also focus on a name and the details of the life story mentioned on the back of the card to start genealogical research.
Note that the illustration and the note on the back may not always have an obvious connection.
Sources:
https://www.lanouvellerepublique.fr/a-la-une/un-document-une-histoire-la-carte-postale
https://www.hurgon.fr/cartes_postales/histoire_de_la_carte.htm
https://museedelacartepostale.fr/origines-carte-postale/
https://fr.wikipedia.org/wiki/Carte_postale
https://philapostelbretagne.wordpress.com/2017/03/24/dominique-piazza-inventeur-de-la-carte-postale-photographique/
http://memoiredhistoire.canalblog.com/archives/2019/09/14/37635584.html
https://www.speciclass.com/page/24-conserver-archiver-photos
https://www.cparama.com/forum/quelques-mises-en-garde-pour-conserver-vos-cartes-t1402.html