信件
xìn jiàn · HSK 7
letter
✉️ 信 (letter/message) + 件 (item/piece) = a written letter, an item of mail
信 carries the sense of message or correspondence, and 件 is a counter-like word for items or documents. Together 信件 names a physical letter sent by mail. Sound: xìn jiàn, with xìn sharing the falling tone of the related word 信 for trust.
信 — person 亻 + speech 言
信 combines 亻(person) with 言(speech), depicting a person standing by their words. The original sense is 'truthfulness' or 'sincerity'—what a person says can be trusted. From this it extended to 'believe,' 'trust,' and later 'message' or 'letter,' since a letter carries one's word.
件 — person radical 亻 + ox 牛
件 combines the person radical 亻 with 牛 (ox). Originally it referred to dividing or counting items, with oxen being valuable units of property to count. It came to mean a measurable item or matter, and today serves as a measure word for things, documents, and affairs.