Postcard research

I don't think I've ever received a postcard, or bought one that wasn't from kitschy tack loaded location, Greetings from Scarborough, etc. Anyway for this task this was just to get an idea of what Postcards are like  visually and traditionally. However what is a postcard, well by the oxford dictionary definition it's:

noun

• a card for sending a message by post without an envelope, typically having a photograph or other illustration on one side.
  
  
  Obviously this allows for some sway artistically for what I want to create using the dimensions of a postcard, which are often 6" x 4", but any size around there is allowed also.