What Does the Expression “Read Between the Lines” Mean and Where Did the Idiom Come From?

Sometimes the truth is obscured within the written text of a letter, and so we must “read between the lines.”

Centuries ago it was discovered that by writing a secret message between the lines of a normal letter with lemon juice, the real message would stay transparent until the document was heated over a flame.

The heat from the flame causes the juice to become discolored, revealing the intended message written between the lines of the ruse.