What's the difference between at an office and in the office?

[Rashed2002]

What's the difference between at an office and in the office?

[Mister Micawber]

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at an office = at the point location of any place of business where professional or clerical duties are performed.

in the office = inside a specific 3-dimensional place of business where professional or clerical duties are performed.
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Native English teacher at Mister Micawber's