The word "等于" is formed by combining two characters: "等", which originally depicted bamboo strips of equal length used for writing and carries the core meaning of "equal" or "grade", and "于", a classical Chinese preposition meaning "in", "at", or "to", which here functions to link and point toward a state or result. Their fusion creates a conceptual bridge where "等" establishes the notion of equivalence, and "于" grammatically directs that equivalence toward something else, resulting in the modern meaning of "to be equal to". Thus, it is a logical compound where the second character structurally enables the first character's meaning to be applied in a relational sense.