The rule says that if a singular noun ends in consonant + "o" then the plural form will be consonant + "oes".

e.g. tomato => tomatoes.

Then, why this rule does not apply to piano?

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    The exception proves the rule. "Piano" is a word of Italian origin and Italian words ending in "o" form the plural by adding only "s". The same for "casino" casino - casinos – Centaurus Jul 21 '17 at 16:06
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    Because the First Rule in the English language is that no rule works 100% of the time. And when I say "no rule" I include the First Rule... – AndyT Jul 21 '17 at 16:07
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    The rule is wrong. English spelling is chaotic and has to be learned individually. Don't trust any English spelling rules; there will always be many exceptions (and anyone who states the rule will never tell you all of them, because they don't know, either). – John Lawler Jul 21 '17 at 16:15
  • Geese, moose, sheep to name a few other random pluralisations – marcellothearcane Jul 21 '17 at 17:53
up vote 1 down vote accepted

Plural of nouns ending in -o:

Nouns ending in -o can add either -s or -es in the plural, and some can be spelled either way.

As a general rule, most nouns ending in -o add -s to make the plural:

So you have:

solo solos, zero zeros, avocado avocados

Note that:

Those which have a vowel before the final -o always just add -s:

like:

studio studios, zoo zoos, embryo embryos.

So:

piano, pianos

Oxford Dictionary

  • 5
    You just missed answering the question and still it is accepted - that's bizarre. "Those which have a vowel before the final -o always just add -s like studio studios etc." So piano pianos? How is n a vovel? The link thankfully is correct and it has specific examples of words ending with a consonants followed by a vowel using es as suffix for plural, like domino dominoes. – Nikhil Girraj Jul 26 '17 at 17:07

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