This is a very basic guide to give some simple pointers to help determine the age of standard sized (3.5 inch X 5.5 inch) postcards. While it may not always be possible to determine the exact age of a card, you can usually come up with an accurate estimate.
The first step is to determine the type of card you have: chrome, linen, white border, early 20th century or Real Photo Postcard (RPPC).
Chromes were usually made from glossy color photographs, and typically range from the 1950s-1970s. They don't all fall into this date range, I've seen them as early as 1939 & as late as the 1990s. But the vast majority are from the decades of the 1950s, 60s & 70s.
Linen cards were produced from about 1933 to the early 1950s. These cards are printed on rough card stock with a linen texture.
White border cards were produced from around 1919 to 1932, generally have low contrast pale colors, which do not go all the way to the edge. This leaves a white border around the picture.
Early 20th century cards are cards produced before 1919.
RPPC are frequently (tho not always) one of a kind photographs with a postcard back. Many are from the early 20th century, but they can be recent. When I think of an RPPC, I'm usually thinking of a card that is not mass produced. The era the cards are from can usually be determined by the stamp box on back (AZO, EKC, etc). There are too many types & variations to list here, but you should be able to find them easily by doing an online search.
Other common sense things you can look for:
Before 1907, postcards had undivided backs. By law, you could only write the address on the back, any messages had to be written on the picture side. (I think this changed after March 1, 1907)
If an address (such as distributor or publisher) has a two digit city code (such as New York 16, NY), you can date the card to between 1943 & 1963.
If an address has a zip code, it dates 1963 or later
If an address has a zip code + 4, it dates to 1983 or later
Phone numbers can give a clue. Something like HUdson 2-5555 is an old number, probably dating to the 1960s at the latest.
If a phone number is 4 digits or less, its quite old
If a card has a postmark, it was obviously created sometime before the cancellation date. However, cards can be mailed years or decades after they were created.
If the card has a stamp box that says "Place 1 cent stamp here", it was created before Jan 1, 1952. Rates were permanently raised to 2 cents on that date.
If the stamp box says "1 cent USA, Canada, Mexico, Cuba, 2 cents foreign", it's probably early 20th century.
If an address has a non-standard state abbreviation, such as S. Dak for South Dakota, it's generally older.
Sometimes cards have a copyright date.
You can estimate dates by clothing styles, cars, city skylines, street scenes, signs and many other things
Some times captions will give dates.
Many cards have catalog numbers on them, if you can get the manufacturers catalog information, you can find out exactly when it was created.
This is very basic, but hopefully it gives you an idea of some things to look for when trying to figure out how old a postcard is.
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