(Writing Systems, An introduction to their linguistic analysis; Florian Coulmas)
Take care of the sense, and the sound will take care of themselves.
It is a recognized fact, and has been for millennia, that there are two complementary classes of speech sounds, consonants and vowels. Segmentalism, we noted in the previous chapter, is a view of language that treats both classes exactly alike, inspired to do so, perhaps, by interpreting the Graeco-Latin alphabet as an iconic map of speech sounds where letters order represents the sequence of articulated sounds. As a matter of principle, letters for vowels and consonants are assigned equal space in writing system derived from the Greek alphabet, and as a class V letters are indistinguishable in form from C letters. Indeed, the equation of both is usually quoted as the crucial accomplishment of Greek writing. Yet, there are some conspicuous differences between vowels and consonants.
Differences between consonants and vowels
(...) vowels have syllabic status, consonants usually do not. Certain consonants such as /n/, /l/ and /w/ are syllabic in some languages and, on the other hand, there are also non-syllabic vowels, but generally speaking syllabicity is more closely associated with vowels than with consonants. Vowels rather than consonants bear stress accent, pitch and tone.
Another difference between the two classes of speech sounds is seen in their relative volatility. On the stage of sound change vowels play the leading roles, while consonants are usually assigned walk-on parts. In the following examples of the Great Vowel Shift from Old English (OE) to Modern English (ME) in the fifteenth century, the vowels changed, but the consonants were preserved.
Vowels are also more prone to get changed in paradigmatic derivations such as 'foot', 'feet', 'tooth', 'teeth', 'mouse', 'mice', 'breathe', 'breath', 'sit', 'sat'. Consonants from the skeleton of these words, vowels provide the flesh. (...) To be sure, consonants too change over time, but vowels are of their nature more unstable, especially long vowels, which often end up being diphthongs, as in 'house'.
Consonants and vowels differ in production and sound quality. Vowels are pure sing-song that opera singers can use when practising their voice, a e i u ae o. Vowels are more likely than consonants to be uttered in isolation and to form words and interjections. In vowels the air stream released from the lungs is uninhibited, differences between them resulting from modifying the shape and length of the resonant cavities that the sound passes through. By contrast, consonants are hisses, hums, buzzes and puffs of air forced through the relatively constricted, or completely closed, vocal tract. They are produced by moving the tongue, the most important of the speech organs, around the mouth, obstructing the flow of air in various ways or bringing it to a stop by closing the lips. Though consonants cab be produced in isolation, ssssss, krrrr, they are typically articulated in combination with a vowel, pooh, ugh.
Yet another difference between consonants and vowels concerns their distribution and function in different languages. Most languages have a clear division of labour between vowels and consonants: syllables begin with consonants and end with vowels; clusters are rare. These are general tendencies, but there are marked differences on other levels of the language system. Some languages rely more heavily on consonants than others.