Both are used, but "clearer" has the advantage of being grammatical.

clear = einsilbig - daher Steigerung: clear, clearer, clearest

to see sth. more clearly
z.B. ist was anderes

Grammatical? Bedeutet das automatisch grammatikalisch korrekt?

For most monosyllabic adjectives (that do not have irregular comparative forms) there is an analytic (periphrastic) and an inflectional form, and the inflectional form is much more usual.

In the case of "clear", the analytic form seems relatively common (though less common than the inflectional form).

I would guess that the analytic/periphrastic form ("more clear") is (for instance) used more in combination with other adjectives that require the analytic form, e.g.
"make the images clearer and bigger"
but
"make the statement more clear and compelling".

A few monosyllabic adjectives (e.g. participial adjectives like "worn") do not have a common inflectional form.

re 'more clear and compelling'

This could be interpreted as a complex adjective 'clear and compelling' whose comparative would naturally be formed with 'more'.

Test: is there any difference between 'more clear and compelling' and 'clearer and more compelling'? I think there is, at least subjectively.

I would not go so far as to call 'more clear' ungrammatical. There's a sliding scale: 'more big' is decidedly off limits; 'more weird' is marginal, and 'direr' is strange.

However, both M-W and American Heritage Dictionary show:

dire, direr, direst

Thanks, everyone! A big help as always.

dire, direr, direst: This one even has a fourth form: direstraits

;-)

Bear in mind that "most + adj" can also be used in an absolute rather than comparative manner:

The interview was most unpleasant.

An -est form could not be used in the above example.

#5
I agree with the sliding scale.
"More good" is wronger than "more big", which is wronger than "more weird". "Gooder" is, of course, wronger than "wronger".

I was wondering about the complex adjective. I think it makes the use of "more" more acceptable but I don't think it explains everything. I don't think you would write "more good and beautiful", but you might get away with "more big and beautiful".

It's true that there are degrees of awfulness, but nevertheless, many of us would simply say that 'clearer' is right and 'more clear' is wrong.

There was a discussion on this very point (Decline and fall of grammatical standards #286.93, or similar) in some previous thread (maybe started by Amy-MiMi?) that I can't locate at short notice.

But in general, if a one-word comparative or superlative exists at all, then it should be used.

Zu #11:

'Commoner' used to be commoner, but 'more common' is now more common.
(Zit. nach Michael Swan, Practical English Usage.)
:-)

OT: Anlässlich eines aktuellen Popsongs: Was sagt ihr zu "shallower"?

I have no problem with "narrower", therefore "shallower" is OK, too, IMO :-)

A Comprehensive Grammar of the English Language (Quirk et al.) has:

"Real, right, wrong . . . take only periphrastic forms. However, most other monosyllabic adjectives take either inflectional or periphrastic comparison.
. . .
Most adjectives that are inflected for comparison can also take the periphrastic forms with more and most. With more, they seem to do so more easily when they are predicative and are followed by a than clause:
John is more mad than Bob is.
It would be difficult to find a man more brave than he is.
. . ."

MikeE:

Pardon me if I have misunderstood, but are you and your source saying that "madder than" and "braver than" are inferior to the quoted examples?

I think it must be "madder than" and "braver than."