The word 写信 is formed by directly combining two meaningful characters: 写 (xiě), which depicts the action of writing, and 信 (xìn), which represents the object, a letter or message. It is a conceptual composite where the first character denotes the action and the second character denotes the target of that action, creating a complete, intuitive idea, the act of writing a letter, through the simple, logical juxtaposition of these visual and semantic components.