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I've come across two spellings for this word. Siphon and syphon are apparently both correct. English is not my first language and this word is not used often in practice, especially in written form. I am very curious about this. Is there any reason to prefer one over the other? And which spelling would be more common? Is it a regional thing (like maybe American like one and British the other) or something else. I wonder if this is a specific case of more common interchange between i and y in some words - which would be interesting.

Wikipedia suggests that siphon may be more common spelling.

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A Google Ngram suggests that it's five times as popular overall (though this wasn't always the case). – Edwin Ashworth Mar 26 '14 at 19:57
Oh cool, maybe syphon is the older version – Chemist Mar 26 '14 at 20:09
Unlikely, as it seems not to be mentioned by the Online Etymology Dictionary. – Edwin Ashworth Mar 26 '14 at 20:23
Very good question! The spelling with a y is completely unetymological and doesn’t seem to occur anywhere but English—even Dutch (where long i gives ij, which is often confused with y in writing) has sifon with an i. It seems at some point in the history of the English language, someone just decided this word would look nicer with a y than with an i, and the spelling stuck. – Janus Bahs Jacquet Mar 26 '14 at 21:02
The "ph" makes it look Greek in origin, and the "y" seems to go well with that look. E.g. "polyphony", "symphony". "Python" also comes to mind... – Michael Kay Oct 9 '15 at 6:59
up vote 1 down vote accepted

Both are in the "Oxford Advanced Learners Dictionary" so I don't think there's any strong reason to prefer one over the other.

As a native speaker of English I would say 'siphon' is the more commonly used spelling (for example see this Guardian article: http://www.theguardian.com/science/blog/2010/may/10/dictionary-definition-siphon-wrong).

I personally find 'syphon' much more aesthetically pleasing though.

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