F/B: front/back harmony
R|T: rounding, total harmony
—: not applicable

Erzya

If we consider that there is an /o/ : /e/ alternation, we could also expect an /u/ : /i/ alternation (which, instead of or in addition to the /o/ : /e/ alternation, actually exists in some dialects).   Although if we consider that the vowels /u/ and /i/ (similarly to /o/ and /e/) are not a perfect alternating pair (they differ not just in F/B, but also in rounding), we could conclude that the /u/ : /i/ alternation is impossible because they are not a good candidate for an alternating pair. However, due to the fact that /u/ is in principle prohibited in non-initial syllables, and the dialects show that the difference in rounding do not exclude the alternation between high vowels either, it seems to be more convincing to analyse /i/ and /u/ as a potential alternating pair (at least in Erzya).