"Read between the lines"

I recently wrote a term paper about "steganography", the practice of secretly hiding a message inside another seeminingly innocent message. A prime example of steganography is the use of invisible inks. The most common form of invisible writing from ancient times was the use of invisible inks, which darken when heated. This allowed writing hidden messages "between the lines" of an innocent letter. Common sources for invisible inks were milk, juices and even urine. Perhaps this is where the term "read between the lines" was developed.....justa thought

Interesting.

Agreed, but my researches give a mild variation, hopefully helpful:

To read between the lines is to be able to discern a hidden or secret meaning; to draw conclusions which are not at first apparent. The background here is that of cryptography where one method of secret writing was to position the message in such a way that it was only intelligible when alternate lines were read.