Pronunciation of “er” versus “ère” changes with number of syllables?

Question

Is it a fair assumption that all monosyllabic adjectives ending in “er” are pronounced the same when converted to the feminine “ère” ending? “Cher”and “chère”, for example? Whereas multi syllable “er” adjectives must change pronunciation in the feminine? “Dernier” versus “dernière”?

I always assumed that the accent grave was added to certain words to change the stress and pronunciation. If so, why is the accent grave added to words like “Cher” in the feminine if the pronunciation does not change?

Does this question make sense?

Thank you!

Accepted Answer

Your question makes perfect sense, but it's not easy to answer. It's not true for all multisyllabic words, because there is also "amer/amère". The difference is that the final "r" is pronounced in "amer", but not in léger". 
If a word ends in -e + consonant, and the consonant is pronounced, I think you will always get an open "e" sound (i.e. "è") , and the grave accent is not required. So there is also "la mer", "le fer", "le sel", "l'hôtel" etc. (Can anyone suggest any exceptions?)
When there is a vowel after the consonant, this rule doesn't apply, so you get "chère", "la mère" etc. instead. I suppose, logically, there must be some words ending in "-ere", without a grave, where the "e" is either closed (like "é") or a schwa ("uh") sound, but I can't actually think of any.

Answer

I guess the question is rather, why is the accent grave left off from cher. By all rights, cher should sound like j'ai. The same happens, e.g., for fier/fière.