There exist 16 diphthongs in the Lojban language. A diphthong is a vowel sound that consists of two elements, a short vowel sound and a glide, either a labial (IPA [w]) or palatal (IPA [j]) glide, that either precedes (an on-glide) or follows (an off-glide) the main vowel. Diphthongs always constitute a single syllable.
For Lojban purposes, a vowel sound is a relatively long speech-sound that forms the nucleus of a syllable. Consonant sounds are relatively brief and normally require an accompanying vowel sound in order to be audible. Consonants may occur at the beginning or end of a syllable, around the vowel, and there may be several consonants in a cluster in either position. Each separate vowel sound constitutes a distinct syllable; consonant sounds do not affect the determination of syllables.
The six Lojban vowels are “a”, “e”, “i”, “o”, “u”, and “y”. The first five vowels appear freely in all kinds of Lojban words. The vowel “y” has a limited distribution: it appears only in Lojbanized names, in the Lojban names of the letters of the alphabet, as a glue vowel in compound words, and standing alone as a space-filler word (like English “uh” or “er”).
The Lojban diphthongs are shown in the table below. (Variant pronunciations have been omitted, but are much as one would expect based on the variant pronunciations of the separate vowel letters: “ai” may be pronounced [ɑj], for example.)
Letters IPA Description
ai [aj] an open vowel with palatal off-glide
ei [ɛj] a front mid vowel with palatal off-glide
oi [oj] a back mid vowel with palatal off-glide
au [aw] an open vowel with labial off-glide
ia [ja] an open vowel with palatal on-glide
ie [jɛ] a front mid vowel with palatal on-glide
ii [ji] a front close vowel with palatal on-glide
io [jo] a back mid vowel with palatal on-glide
iu [ju] a back close vowel with palatal on-glide
ua [wa] an open vowel with labial on-glide
ue [wɛ] a front mid vowel with labial on-glide
ui [wi] a front close vowel with labial on-glide
uo [wo] a back mid vowel with labial on-glide
uu [wu] a back close vowel with labial on-glide
iy [jə] a central mid vowel with palatal on-glide
uy [wə] a central mid vowel with labial on-glide
(Approximate English equivalents of most of these diphthongs exist: see Section 11 for examples.)
The first four diphthongs above (“ai”, “ei”, “oi”, and “au”, the ones with off-glides) are freely used in most types of Lojban words; the ten following ones are used only as stand-alone words and in Lojbanized names and borrowings; and the last two (“iy” and “uy”) are used only in Lojbanized names.
The syllabic consonants of Lojban, [l̩], [m̩], [n̩], and [r̩], are variants of the non-syllabic [l], [m], [n], and [r] respectively. They normally have only a limited distribution, appearing in Lojban names and borrowings, although in principle any “l”, “m”, “n”, or “r” may be pronounced syllabically. If a syllabic consonant appears next to a “l”, “m”, “n”, or “r” that is not syllabic, it may not be clear which is which:
4.1) brlgan.
[br̩l gan]
or [brl̩ gan]
is a hypothetical Lojbanized name with more than one valid pronunciation; however it is pronounced, it remains the same word.
Syllabic consonants are treated as consonants rather than vowels from the standpoint of Lojban morphology. Thus Lojbanized names, which are generally required to end in a consonant, are allowed to end with a syllabic consonant. An example is “rl.”, which is an approximation of the English name “Earl”, and has two syllabic consonants.
Syllables with syllabic consonants and no vowel are never stressed or counted when determining which syllables to stress (see Section 9).