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
Rheinische Dokumenta two long monophthongs. Varying lengths of diphthongs are not noted in Rheinische Dokumenta, which at least does not create ambiguities within a dialect. There are no triphthongs, although diphthongs can be followed by schwas in some languages. If, for instance, Rheinische Dokumenta was used in writing Westphalian, triphthongs would be written in a manner analogous to the diphthongs, using three adjacent letters of vocals. Since most dialects follow the German, and Lower Franconian, rule of final-obstruent devoicing, voiced consonants cannot, or hardly ever, appear at the end of a word or sentence. This is one of the major differences between