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What is the difference between

"as a side note" and "on a side note"?

Are they interchangeable? Which one is preferred over the other?

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They're interchangeable. I prefer "on", but I freely admit I'm in the minority. –  FumbleFingers Jun 11 '12 at 1:46
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2 Answers 2

up vote 6 down vote accepted

I think "as a side note" makes more sense, given that a side note is literally "a notation made in the margins or sides of a page; also called a marginal note." So when you're adding a bit of additional detail, you can preface it with "As a side note to this information,...."

The NGram for as versus on a side note shows that as a side note was more widely used in English books from 1800 to 2008. (Neither show up much in just the British English corpus.)

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With the similar idioms "on another note" or "on a different note," I would use the preposition on.

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"As a side note" is a metaphorical phrase, and will need to be used as a predicate, or subordinately to a predicate. e.g. "This acts as a side note..." it makes a statement about something. This form is to be equated with something IN the sentence itself.

"On a side note" is a locative, and is to be used more logically in a sentence. As an interjection, it introduces the following sentence which is itself to be considered as secondary/complementary. This form is to be equated with the statement as a whole.

The latter, "On .." should be used to begin a sentence, as an interjection. I imagine this would be your preferred option, as the former really ought to be part of a sentence, and if it is used at the beginning will be a subordinate clause, i.e. "As a side note, the ... is ...".

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I think this distinction is spurious. They're simply two different prepositions for the same metaphorical usage - any given speaker will normally use just one or the other. As it happens, "as" has historically been used far more often, but "on" (which I suspect is more common in Britain) is rapidly catching up. –  FumbleFingers Jun 11 '12 at 1:46
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