broad labial (for example, "book"; domun "world"). The phoneme occurred in other circumstances. The occurrence of the two phonemes was generally unrelated to the nature of the corresponding Proto-Celtic vowel, which could be any monophthong: long or short. Long vowels also occur in unstressed syllables. However, they rarely reflect Proto-Celtic long vowels, which were shortened prior to the deletion (syncope) of inner syllables. Rather, they originate in one of the following ways: from the late resolution of a hiatus of two adjacent vowels (usually as a result of loss of *s between vowels); from compensatory lengthening in response to loss of a consonant (cenél "kindred, gender" < *cenethl; du·air-chér "I have purchased" < *-chechr, preterite of crenaid "buys"); from assimilation of an unstressed vowel to a corresponding long stressed vowel; from late compounding; from lengthening of short vowels before unlenited , still in progress in Old Irish (compare erríndem "highest" vs. rind "peak"). Stress Stress is generally on the first syllable of a word. However, in verbs it occurs on the second syllable when the first syllable is a clitic (the verbal prefix as- in as·beir "he says"). In such cases, the unstressed prefix is indicated in grammatical works with a following centre dot (·). Orthography As with most medieval languages, the orthography of Old Irish is not fixed, so the following statements are to be taken as generalisations only. Individual manuscripts may vary greatly from these guidelines. The Old Irish alphabet consists of the following eighteen letters of the Latin alphabet: a, b, c, d, e, f, g, h, i, l, m, n, o, p, r, s, t, u, in addition to the five long vowels, shown by an acute accent (´): á, é, í, ó, ú, the lenited consonants denoted with a superdot (◌̇): ḟ, ṡ, and the eclipsis consonants also denoted with a superdot: ṁ, ṅ. Old Irish digraphs include the lenition consonants: ch, fh, th, ph, sh, the eclipsis consonants: mb, nd, ng; ṁb, ṅd, ṅg, the geminatives: bb, cc, ll, mm, nn, pp, rr, tt, and the diphthongs: aé/áe/aí/ái, oé/óe/oí/ói, uí, ía, áu, úa, éu, óu, iu, au, eu, ai, ei, oi, ui; ái, éi, ói, úi. The following table indicates the broad pronunciation of various consonant letters in various environments: {| class="wikitable" style="text-align: center;" |- !colspan=6|Broad consonant phonemes |- !rowspan=2|Letter !colspan=3|Word-initial !colspan=2|Non-initial |- !unmutated !eclipsed !lenited !single !geminate |- |b | | | colspan=2|