What is the origin of the idiom "read between the lines" and when to use it?

The origin of the idiom "read between the lines" can be traced back to the practice of reading written or printed text. In written communication, the lines of text are usually straightforward and explicitly convey the primary message. However, the subtler nuances, implications, or hidden meanings may require a more attentive and thoughtful reader to interpret. Therefore, "reading between the lines" means examining a text or message for deeper or hidden meanings beyond its surface. It is often used in situations where there may be subtext, hidden agendas, or unspoken intentions.