Context: In Western Catalan, unstressed vowels reduce to five: /e ɛ/ > [e]; /o ɔ/ > [o]; /a u i/ remain distinct. This reduction pattern, inherited from Proto-Romance, is also found in Italian and Portuguese. Some Western dialects present further reduction or vowel harmony in some cases.

Question: What do unstressed vowels reduce to in Western Catalan?
Answer: five

Question: Which letters remain distinct?
Answer: /a u i/

Question: Where did this pattern come from?
Answer: Proto-Romance

Question: In what other languages is this pattern found?
Answer: Italian and Portuguese

Question: What do other Western dialects sometimes  offer?
Answer: further reduction