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At least to a self-professed geek, it's cool to write words like fungii and radii, so naturally, in some informal communications, I take every opportunity to apply the suffix where it's probably not supposed to be used.

Thus I ask, what is (are) the rule(s) for the double 'i'?

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    The rule is replace "us" with "i". fungus→fungi; radius→radii, Prius→Prii (although the last one is technically invalid, since it only works with Latin words of the second declension). – Peter Shor Oct 24 '13 at 19:02
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    It isn't there in fungi; that's a single I. In radii, the first I is part of the root radi- and the second I is the plural nominative suffix -i. The best rule for using Latin plurals in English is to get them right. If you have to ask about the rules, you should study Latin. – John Lawler Oct 24 '13 at 19:06
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    @MDMoore313: As should be obvious by now, there are risks involved if you latch on to "cool" usages without knowing what's right, and why. You surely don't want people to remember you for decades to come as the guy who once told them fungii was a cool word (but some time later they discovered it wasn't even a valid word at all). Me, I think you should go with the future (which means getting used to regular forms such as corpuses rather than stuffy old-fashioned corpora). – FumbleFingers Oct 24 '13 at 20:15
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    And note that it's corpora and not corpi because corpus is a Latin word in the third declension. – Peter Shor Oct 24 '13 at 20:27
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    @WS2: As should be obvious from my previous comment, I'd unquestionably go for referendums. Interestingly, people still think Brits are linguistically "conservative" (simply because we never had anything like Webster's revisions), but the evidence from corpuses suggests we're actually leading the move towards regularising things like dialog, funguses, etc. – FumbleFingers Oct 24 '13 at 20:29
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The ‘i’ is a Latin plural. ‘Fungus’ is the singular and ‘fungi’ (not 'fungii’) is the plural. The letter ‘i’ occurs twice at the end of ‘radii’ only because there’s an ‘i’ in the singular ‘radius’.

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