The word 信封 is formed by combining two characters: 信 (xìn), which means "letter" or "message", and 封 (fēng), which means "to seal" or "an envelope". Together, they create a compound that visually and conceptually represents the object used for a letter, specifically, a container (the envelope) that seals the message inside. It is a direct, logical synthesis where the first element names the content and the second describes the function or form of the container, resulting in the meaning "envelope".