respectively: vs. .) The inventory of Old Irish long vowels changed significantly over the Old Irish period, but the short vowels changed much less. The following short vowels existed: {| class="wikitable" style="text-align: center;" ! ! colspan="2" | Monophthongs ! colspan="2" | Diphthongs |- !Close | | | | |- !Mid | | | | 1 |- !Open | colspan="2" align="center" | , (?) | colspan="2" align="center" | |} 1The short diphthong likely existed very early in the Old Irish period, but merged with /u/ later on and in many instances was replaced with /o/ due to paradigmatic levelling. It is attested once in the phrase by the prima manus of the Würzburg Glosses. arose from the u-infection of stressed by a that preceded a palatalized consonant. This vowel faced much inconsistency in spelling, often detectable by a word containing it being variably spelled with au, ai, e, i, or u across attestations. "hill, mound" is the most commonly cited example of this vowel, with the spelling of its inflections including tulach itself, telaig, telocho, tilchaib, taulich and tailaig. This special vowel also ran rampant in many words starting with the stressed prefix air- (from Proto-Celtic *ɸare). Archaic Old Irish (before about 750) had the following inventory of long vowels: {|class="wikitable" ! ! colspan="2" | Monophthongs ! colspan="2" | Diphthongs |- ! Close | | | | |- ! Mid | 1 | , (?)2 | | , ()3 |- ! Open | colspan="2" | | colspan="2" | 3 |} 1Both and were normally written é but must have been pronounced differently because they have different origins and distinct outcomes in later Old Irish. stems from Proto-Celtic *ē (< PIE *ei), or from ē in words borrowed from Latin. generally stems from compensatory lengthening of short *e because of loss of the following consonant (in certain clusters) or a directly following vowel in hiatus. It is generally thought that was higher than . Perhaps was while was . They are clearly distinguished in later Old Irish, in which becomes ía (but é before a palatal consonant). becomes é in all circumstances. Furthermore, is subject to u-affection, becoming éu or íu, while is not. 2A similar distinction may have existed between and , both written ó, and stemming respectively from former diphthongs (*eu, *au, *ou) and from compensatory lengthening. However, in later Old Irish both sounds appear usually as úa, sometimes as