The study and collecting of postcards is termed "deltiology".

It is possible to form a large collection of postcards around the theme of parrots and parakeets which were very popular subjects, especially on the early comic or saucy postcards. At the peak of their popularity, in the 1930s, sales of saucy postcards alone reached a massive 16 million a year.

Curt Teich Invitation Postcard

This colourful postcard is from the Curt Teich company of Chicago, USA, published in 1938. Put out the cat, pull the blinds, lock up the doors and come to . . . while the parrot asks "where we going?".

There is a code on Curt Teich postcards which can be translated to give the date of printing. Inside the box where we are asked to place a one cent stamp (the good old days!), there is a tiny code "8A-H2134". The letter "A" indicates the 1930s and the "8" shows that it was 1938. A more detailed explanation can be found here.