DIPHTHONG,MONOPTHONG AND SILENT LETTER.

A diphthong from GreeK means diphthongos, literally "two sounds" or "two tones"), also known as a gliding vowel, is a combination of two adjacent vowel sounds within the same syllable. Technically, a diphthong is a vowel with two different targets: that is, the tongue (and/or other parts of the speech apparatus) moves during the pronunciation of the vowel. In many dialects of English, the phrase no highway cowboys /ˌnoʊ ˈhaɪweɪ ˈkaʊbɔɪz/ has five distinct diphthongs, one in every syllable.Diphthongs contrast with monophthongs, where the tongue or other speech organs do not move and the syllable contains only a single vowel sound. For instance, in English, the word ah is spoken as a monophthong (/ɑː/), while the word ow is spoken as a diphthong in most dialects (/aʊ/). Where two adjacent vowel sounds occur in different syllables—for example, in the English word re-elect—the result is described as hiatus, not as a diphthong.Diphthongs often form when separate vowels are run together in rapid speech during a conversation. However, there are also unitary diphthongs, as in the English examples above, which are heard by listeners as single-vowel sounds (phonemes).Diphthongs use two vowel sounds in one syllable to make a speech sound. A monophthong is a pure vowel sound, one whose articulation at both beginning and end is relatively fixed, and which does not glide up or down towards a new position of articulation. The monophthongs can be contrasted with diphthongs, where the vowel quality changes within the same syllable, and hiatus, where two vowels are next to each other in different syllables. A vowel sound whose quality does not change over the duration of the vowel is called a pure vowel.The conversions of monophthongs to diphthongs (diphthongization), and of diphthongs to monophthongs (monophthongization), are major elements of language change and are likely the cause of further changes.In some languages, due to monophthongization, graphemes that originally represented diphthongs now represent monophthongs. Silent letter In an alphabetic writing system, a silent letter is a letter that, in a particular word, does not correspond to any sound in the word's pronunciation. Phonetic transcriptions that better depict pronunciation and which note changes due to grammar and proximity of other words require a symbol to show that the letter is mute. Handwritten notes use a circle with a line through it and the sound is called "zero"; It resembles the symbol for the "empty set", but must not be confused with the Danish and Norwegian letter Ø. In printed or computer's graphic presentation, using the IPA system, the symbol ∅ is used.