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I often see 'commoner' and according to dictionaries it's correct. Sometimes I also see 'more common'. According to the grammtical rules 'commoner\commonest' are unacceptable, aren't they?
Thanks
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No. Many two syllable adjectives/adverbs offer you the option. Even for one syllable words, the 'more/most' form can be used for emphasis.
What do you mean 'option'? The option of being used for emphasis?
But if it's not correct grammatically how can it be used just for emphasis,Mr.M?

Can you illustrate any one-syllable word which can be preceded by 'most\more'?

Or I misunderstood you?
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I don't know how you could misunderstand me, Mav; I couldn't make myself more clear if I were Francis Fowler! An option is a valid choice:

This is the most plain and direct way of explaining the point.
Pens are more common than swords in my house; in fact, they are the commonest pointed object.
So both 'more common' and 'commoner' /AND/ 'more common' and 'commonest' are possible?
Yes indeed. Did you try googling? Let's:

more common - 6,630,000
commoner - 281,000
most common - 18,600,000
commonest - 337,000

Yipes! Rather lopsided preferences; but still all possible, I think. I do not change my opinion, because I do in fact use all of them myself.
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Yeah I see. Thank you, MrM.