Two-syllable adjectives such as common, polite, quiet often have either type of comparison:

common ~ commoner ~ commonest OR

common ~ more common ~ most common

The endings sometimes involve changes in spelling (see 700, 703) or pronunciation (see 666), for example:

pretty ~ prettier ~ prettiest, big ~ bigger ~ biggest

l Long adjectives (awkward, possible, hopeful, useful, etc.), including -ed adjectives (interested, etc.) and -ing adjectives (interesting, etc.) form comparison with more and most:

I find my new work more challenging and more interesting.