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
Scottish Gaelic phonology types, depending on the status of one or more of the written vowels in the combinations. Category 1: vowel plus glide vowels. In this category, vowels in digraphs/trigraphs that are next to a neighbouring consonant are for all intents and purposes part of the consonant, showing the broad or slender status of the consonant. Category 2: 'diphthongs' and 'triphthongs'. In this category, vowels are written together to represent either a diphthong, or what was in Middle Irish a diphthong. Category 2 digraphs can by followed by Category 1 glides, and thereby form trigraphs: Like the closely related languages, Modern Irish