Commoner or more common

[hristov]

The question is: How to use the ajective 'common' in comparative and superlative degree?
According to Oxford Advanced Dictionary-5th ed. - common- adj. (-er, -est), but some days ago I saw an exersice 11.2 in 'Natural English'-Elementary I think- see Language reference - eleven . Here is the line 'In cities, flats are more common than houses.'
Tell me more abot that, please! Thanks!
Rumen Hristov

[Casiopea]

Both.

Adjectives with two syllables tend to vary. Some take either form, and the situation determines the usage. For example, one will see commoner and more common, depending on which sounds better in the context. Here are four more examples.

By the way, adjectives of two or more syllables that end in "ow" and "le" also take the "er" and "est" suffixes, e.g., narrower, gentler. Oh, and let's not forget the comparative and superlative of often.

On the flipside, shorter words usually take on the suffixes -er/-est but occasionally they may be given the more/most forms for emphasis or other reasons;e.g., Which of them is the most clear?.

All the best.

[Tdol]

And -er: clever/cleverer/cleverest