Vowel whose quality does not change over the duration of the vowel is called a monophthong. Monophthongs are sometimes called "pure" or "stable" vowels. A vowel sound that glides from one quality to another is called a diphthong, and a vowel sound that glides successively through three qualities is a triphthong. All languages have monophthongs and many languages have diphthongs, but triphthongs or vowel sounds with even more target qualities are relatively rare cross-linguistically. English has all three types: the vowel sound in "hit" is a monophthong , the vowel sound in "boy" is in most dialects a diphthong , and
Lojban permitted variants intended to cover dissimilitude in pronunciation by speakers of different linguistic backgrounds. Stress normally falls on the penultimate syllable. There are 16 diphthongs (and no triphthongs). A distinction between diphthongs and monophthongs can be written by inserting a comma in the Latin alphabet. Vowel hiatus is also prevented by inserting an apostrophe, which usually indicates , though there are other valid realizations. For those who have trouble pronouncing certain consonant clusters, there is the option of adding vowels between them (epenthesis), as long as they differ sufficiently from the phonological vowels and are pronounced as short as possible.