built at/in the beginning of last century.

  • Egmont

    Senior Member
    English - U.S.
    "The beginning of the last century," even though one can think of it as lasting several years, has the structure of referring to a single instant in time: the beginning of a trip, the beginning of a meal, the beginning of a person's life. In referring to a single instant in time, we use "at." We cannot use "in:" An instant has no duration, so nothing can be inside it.

    If you were to use another term to describe an interval - "the early part of the last century," "the first decade of the last century," or something like that - then "in" would be correct.
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