The word 信件 is formed by combining two characters, each contributing to the overall meaning. The first character, 信, originally depicted a person (人) next to spoken words (言), conveying the core idea of a "message" or "trustworthy communication". The second character, 件, often signifies a tangible item or a piece of something. When joined, they create a compound where 信 provides the core concept of a personal message, and 件 classifies it as a concrete, countable object, thus arriving at the meaning of "letter" or "correspondence" as a physical item.