Read Between the Lines

Rooted in real cryptographic practice. One early method of sending a secret message was to write it in invisible ink between the visible lines of an ordinary-looking letter, ink that would only appear once warmed over a flame. In times of political danger, when intercepted letters could implicate the sender, hiding the real message this way kept it safe from anyone but the intended reader. A related theory holds that some coded letters were written so that only every second line, read in sequence, revealed the true message. The figurative sense, understanding an unstated meaning in ordinary writing or speech, developed through the 19th century.