介于
jiè yú · HSK 7
between
↔️ 介 = interpose/lie-between + 于 = at; 'to lie between' two points
介于 A 和 B 之间 means to fall between A and B. 介 carries the sense of coming between or mediating, and 于 marks the location or range, so together they pin a value or position to the span separating two endpoints.
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.