“Read between the lines” means looking beyond the words on a page—or the words someone says—to infer an unstated meaning. The expression was in figurative use by the mid-19th century, but its exact first appearance and the person who coined it are not securely established. The surviving evidence points to a period, not a single moment of origin.
The phrase’s image is easy to picture. For centuries, people have used invisible inks to conceal writing, sometimes placing a secret message between the visible lines of an ordinary letter. Heat or a chemical treatment could make the hidden words appear. Such methods give the phrase a plausible connection to secret writing, but plausibility is not proof: no firm evidence shows that this technique gave rise to the expression.
A popular origin story presents the phrase as a direct reference to spies hiding messages in correspondence. Secret writing was real, and writing between visible lines was one possible way to conceal text, but the phrase’s documented history does not point to a particular spy, conflict, or method. It may instead have grown naturally from the broader idea that a reader can find meaning that is not stated outright. Without an early record explaining the wording, a precise origin would be guesswork.
By the 19th century, the phrase was useful far beyond cryptography. Readers could apply it to a writer’s hints, a speaker’s evasions, or the implications of a situation. Today, someone might say, “The manager praised the proposal, but I read between the lines and realized she had serious reservations.” Or, “The announcement never said layoffs were coming, but employees read between the lines.” The phrase endures because people are always trying to make sense of what is suggested, withheld, or left unsaid.