Every back vowel has it's counterpart among front vowels.
Originally a word and it's suffixes could have either back vowels or front vowels only and never both of them.
In practical terms, we might say that, if a word stem has any back vowel, then every suffix added to it may have back vowels only and vice versa e.g. levvettägō vs. ve̮te̮ttagō[2].
i is an exception, because it's counterpart occurres only in very recent Russian loanwords [3].
CERTAIN SUFFIXES DO NOT FOLLOW VOWEL HARMONY RULES.
ö is found in the first syllable only, excluding some Ingrian and Finnish loanwords and derivation suffixes [5]. Thus the following suffixes have no front vowel alternative:
Front vowels were finally preserved only in the second syllables before they were completely replaced with their back counterparts e.g. käsku ('order, demand' in Lüganuse dialect < *käskü) or süö-Du ('eaten' in Lüganuse dialect < *süö-Dü).
e may have substituted ä in some words [8] e.g. lüpseDä ('to milk (a cow)' in Lüganuse dialect < lüpsäDä).
ä may have falsely substituted a following a syllable, which includes e or i, even though the stem could have back vowels [9] e.g. ammasteGä ('with teeth' in Lüganuse dialect), e̮liväD ('they were').
ä may have substituted even e of the second syllable, if the first syllable includes ä or ü[10] e.g. üläs ('upwards' in Lüganuse dialect < üles) or pǟlä ('onto' in Lüganuse dialect < *pǟlle).
ü may have been substituted by i in the second syllable of a stem, if the first syllable includes a short ü e.g. tüli[11] ('quarrel' in Lüganuse dialect) or püssiD ('guns' in Lüganuse dialect < *püssüD ).
In Lüganuse dialect only the a | ä opposition remained active [12] e.g. jättäGä ('Leave (it)!') vs. tuoGa ('Bring (it)!') or tüöGä ('with work') vs. maaGa ('with earth').
The u | ü opposition has left only rudiments [13] e.g. küsütti ('it was asked') or kinniDettüD ('fixed').
