I remember that, and meant to comment it, but obviously forgot.
For me, they are postcards. I have bought a set from second hand store, and as I remember, the package said “IKEA kort” -> card.
Many cards sold in other shops have equally blank writing side.
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I’d say a postcard is everything that can be sent through the mail without an envelope and with the message openly displayed for the postman to read 
The weirdest postcard I ever received was actually a shoe, used as a postcard. The post delivered it, so YES, I’d say it’s a postcard:
However, if a user points out in her/his profile, that she/he wants only “real postcards”, I would only send a storebought postcard, sized between 9x13 and 13x18 cm, with an appointed space for the address marked on it. That is, I guess, the thing that EVERYBODY will (have to) agree IS “a real postcard”. And, of course, I’d send it written & stamped, that is WITHOUT an envelope.
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Loads of my postcards that I send are blank on the back before I write on them (i.e. no lines for address, says the word ‘postcard’) but they are a postcard size and to all intents and purposes still a postcard.
I’ll say this: you can’t please everyone. There’s bound to be a few people who aren’t happy with what you send them, but that’s not for you to worry about as long as you’ve fulfilled writing a message, putting correct postage on it, ID and sending it.
I received a folded greeting card in a tag - it’s a nice card but I was like well, it isn’t a postcard…
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My latest cards I bought from IKEA (images of succulents) aren’t recognizable as such as they are missing the IKEA logo on the back. So far nobody has complained about them. Quite the contrary, they loved them.
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Another kind of a postcard that I would consider to not be a real card is a card sent through a third party service where you don’t even see it yourself. They are real enough to send to a friend but for the purposes of postcrossing I think they are fake and should be firmly banned.
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A postcard is a piece of paper stock, thick enough to safely travel through the mail without an envelope. It typically has an image on one side, and space for the stamps, address and message on the back.
That’s it! Shape, sizes, printing or sending methods, logos or markings don’t change this broader definition — they’re more like personal preferences and details.
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I l.ike Ludek’s definition. That being said, no need to worry, I have no plans to send any shoes in the near future.
While the admin has spoken, I respectfully disagree with her definition to some degree. Paper card stock is one type of material used, but copper, leather and wood have been (and in some cases still are) used as postcard materials. My sister recently sent me a gorgeous porcelain postcard. I would hope that the official guidance would be written so that no one would be discouraged from sending me any of those.
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Still not everything post delivers is a postcard. For me, that is a shoe
But, I hope it’s not a shoe you forgot somewhere, and someone kindly sent it back to you 
although it looks fun!
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We have been discussing this with a focus on handmade cards here:
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Latvia’s Post provides the service “Mail Pigeon”. You can send them a photo by e-mail, they will make a postcard out of it, print the text you wrote and send it to the address of your choice. I haven’t used it, but I think if I got something infectious, I would send my postcrossing postcards exactly this way.
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I remember we had a huge discussion in 2012 about using third party services for the postcards, dua matter of privacy. This happened for London Olympics, when Samsung made free the service “Touchnote” and many Postcrossers used it to send official postcards.
In the Spanish forum we were agree to send those postcards to each other, but no for official Postcrossing purposes. I have around 100 of those Touchnote postcards in a box 
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This is so lovely!.
I’ve seen in th past a few posts with “experiments” made by Postcrossers to see if random items could be sent as a postcard. I specifically can remember a leaf (it was big, no idea from which plant) and also a flip-flop.
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I for one really don’t like it when people use these third-party services, like Touchnote, to send official postcards. I got one from a German member that was a total joke. There is a huge variety of cards available in Germany. It just requires a little effort that this person chose not to do.
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That mailed shoe is not a postcard. It’s a “postcardized” package. Package by size and weight, with outside writing that the Post has tolerated, so the mail piece was accepted for circulation. The postage may or may not be enough for that kind of package.
I have a Facebook group “How to create your own postcards?”.
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I once received a picture cut out of a Magazine accompanied by a letter. That is not a real postcard.
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As we obviously love discussing this question, and I’m a linguist by profession, let me give you a scientific definition of a postcard. And no, that’s not the one you would find in a dictionary!!!
Language has been around long before dictionaries, and even before the concept of a “definition”. So if words have a concrete meaning, this meaning cannot possibly be defined by a definition - because HOW would people have communicated before they came up with the abstract idea of definitions???
The real meaning of words actually cannot be strictly defined in such a way that you can tell in absolute terms whether or not a given word can be applied to a given thing. To use some examples a bit easier - and less controversial 
- than postcards:
- A plane may in poetic terms be referred to as a “beautiful bird” - though usually we all would agree that a bird is an animal and a plane is not.
- While biologists have been explaining over and over again that a whale is not a fish for decades if not centuries, they will NEVER get done with it, as any human child first seeing a whale will describe it as “a big fish”. Somehow its “fishy” appearance just brings the word to mind…
- I once came across a furniture factory that had a gigantic structure placed in front of it - some 20 metres high - which I immediately recognized as “a chair”. Now how could that be possible if the meanings of words were defined by definitions??? By definition, a chair is “a piece of furniture designed to accommodate one sitting or reclining person (…)”. But this thing? No way anybody could ever sit on it or you could put it inside a house! And yet, my wife also thought it was a chair.
Well, the secret behind all this is actually pretty simple. The meaning of a word is stored in our brains as a so-called “prototype” - that is, we all have a concrete picture in our mind of what an X should look like. And then we apply the word to anything that appears similar.
As we all learn our language in different contexts, these prototypes can differ - but they are similar enough for us to be able to communicate…
So my prototype of “a postcard” would be:
a piece of cardboard sized 10x15 cm with a picture on one side and space for writing on the other, with a line dividing the spaces for message and address as well as lines for writing the address on, and a little box indicating where you should put the stamp.
For most of us, I guess, the thing we have in mind when we think “postcard” will be fairly similar. The arguments arise only about HOW MUCH we allow a thing to differ from the prototype to still be willing to call it a postcard. And that, I’m afraid, is a personal choice - and thus a matter that cannot be settled
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I always, when someone plays dumb, like “yes a pen is a postcard”, think, if you put a postcard, a pen, a shoe, a photo and paper to the table, and tell a child to choose the items that are postcards, they choose the postcard.
Yes, dictionaries and definitions are old dated already when they are published.
People who use the language, are most often correct, or changing the language to the direction it one day will be in the dictionary and definitions. (edit. maybe the nowadays children are not so familiar with postcard, but the idea is this).
Still I like the shoe 
it looks like bent blackboard 
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Through the years, I’ve received a few folded cards, cut to become a postcard. However, I keep them with the same nice feelings as if they were real postcards. Not sure if it’s about me as non-native english speaker, that I can understand that maybe in certain languages card and postcard could means the same. But at the same time, I know that’s not something I would send in Postcrossing to another postcrosser.
I found more complicated to digest when I have received an envelope, with a postcard, a printed message and a piece of newspaper. It happened to me a few times now, from different accounts. Not sure if are the same person with multiple accounts. Because even if this person have sent a postcard, there is no sense of personal involving in the process to send the same printed message to several postcrossers.
I still have a few that says “IKEA Kort” and “IKEA Kortebo”.
Last time I went to my nearest Ikea, I found some lovely prints called Yllevad, that shares names with some photo frames.
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IKEA cards are always welcome to me! I have got over 24.000 postcards during 12 years but none of IKEA.
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For me every piece of Postcrossing mail sent is a little gift of the sender. It is nice if someone meets your interests, but more so I enjoy the writing on the back more then the criteria of the definition of “a real postcard”.
Keep also in mind that not every Postcrosser is able to meet all the definitions and demands, especially what you quite often see in the profile of Postcrossers, for many reasons…
So be kind and most of all happy that someone in the world has taken the time and effort to send you a postcard❣
H🙃ppy Postcrossing to you all.
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