The word rarely turns up outside that context.
The word rarely turns up outside of that context.
Which one is correct and why?
The preposition/adverb, outside, and the compound preposition, outside of, both have the same meaning: beyond the boundaries/limits of _
The New Oxford American Dictionary provides an excellent exposition on the usage of these two:
'Outside of' grates on my ear too! The 'of' is superfluous - and I was brought up to believe that brevity is the key to good writing: so unnecessary words should be omitted.
Let's consider two sentences: 1. Outside/Outside of Norway, the world's largest Norse community is in Minnesota. 2. The cemetery is located outside/outside of the town. In these two sentences, the term outside/outside of is used in two different ways. In the first sentence it means "apart from" or "anywhere other than in." Because the synonymous phrase ends with a preposition, the best term here is "outside of." In the second sentence it means "beside" or "alongside." The best term here, I think, is "outside." To summarize, if you could substitute the words "apart from" or other than in," use "outside of." If you could substitute the word "beside" or "alongside," use "outside." I hope this is helpful.
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