It seems unimaginable today to address your mail without a house number and street name, but it was all the rage back in the early 1900s. The postmark on the above postcard says “Address your mail to street and number.” I have countless postcards that are just addressed with a name, street, and the word “city.”
Above is an address with a house number and street name, but no town name, just “city.” Since this postcard is postmarked in Massachusetts, perhaps “city” meant Boston back then. I would guess that “city” stood for the capital or biggest city in the state the postcard is postmarked in.
With the above postcard, we actually get a town name, but no street name & number or state. Notice how it is postmarked in 1908, which is before the postcard above it (postmarked 1916).
As you can see, there was no standard throughout the years for mailing a postcard (and I assume other letters). The “address your mail to street and number” postmark is from 1921. This is around the time they started the crackdown on proper addresses. Even still, addresses were sans zip code. Zip codes didn’t start until 1942, and when they did they were only 2 digits. 5 digit zip codes started in 1963.