在于
zàiyū · HSK 4
in
📍 在 (be at) + 于 (in/at) = lies in, consists in
在于 stacks two location particles meaning be-at + in. Used abstractly: the key lies in X, the problem consists in Y. Both characters carry the at/in sense, doubling for emphasis on where something resides.
在 — earth 土 + phonetic 才
Originally a phono-semantic compound: 土 (earth) gives the meaning of a place or location, while 才 supplies the sound. It came to mean 'to be present/located at,' and now also serves as a grammatical marker for ongoing actions, like 'in the middle of doing.'
于 — two horizontal strokes 二 + vertical hook 亅
于 originated as a pictograph of breath or sound flowing outward, depicting exhalation through a passage. In oracle bone script it resembled a curved line showing motion or extension. Over time it became a grammatical preposition meaning 'at,' 'in,' or 'to,' losing its original concrete sense.