Etymologiae, Book I, chapter 4: De litteris Latinis
Chapter IV. On the Latin letters The nymph Carmentis first handed the Latin letters to the Italians. She was called Carmentis because she sang (canebat) the future in verses (carminibus). Her proper name, however, was Nicostrata. Letters are either common or liberal. They are called common because many use them in common, as to write and to read. Liberal, because only those know them who compose books and know the method of speaking correctly and of dictating. The mode of letters is twofold: for they are divided principally into two parts, vowels and consonants. Vowels are those that are uttered in various ways with an open aperture of the throat, without any collision. And they are called vowels (vocales) because they fill the voice (vocem) by themselves and make a syllable by themselves with no consonant attached. Consonants are those produced by a different movement of the tongue or pressure of the lips. And they are called consonants because they do not sound by themselves, but sound together (consonant) when joined with vowels. These are divided into two parts: semivowels and mutes. Semivowels are so called because they have something half (semis) of the vowels. For they begin from the vowel E and end in a natural sound. Mutes are so called because they do not burst forth except with vowels subjoined to them. For if you take away from them the last sound of a vowel, only the enclosed murmur of the letter will sound. The ancients called vowels, semivowels and mutes “sounding,” “half-sounding” and “soundless.” Among the vowels, I and U have various significations among the grammarians. For they are sometimes vowels, sometimes semivowels, sometimes intermediate. They are vowels because, set alone, they make syllables and are joined with other consonants. They are held to be consonants because they sometimes have after them vowels set in the same syllables, as in “Ianus,” “vates,” and are counted as consonants. They are called intermediate because by nature, alone, they have a middle sound, as in “illius,” “unius.” Joined with others they sound fuller, as in “Ianus,” “vanus”; for alone they sound one way, joined another. I is sometimes called double because whenever it is found between two vowels it is counted as two consonants, as in Troia; for its sound is doubled there. The letter V likewise is sometimes nothing, because in places it is neither vowel nor consonant, as in quis. It is not a vowel, because I follows; it is not a consonant, because Q precedes. And therefore when it is neither vowel nor consonant, it is without doubt nothing. The same is called digamma by the Greeks when it is joined to itself and to other vowels: it is called digamma of the lips because it is double like the letter F, which has a double Gamma, and the grammarians wished the vowels joined in its likeness to be called digamma, as in votum, virgo. Among the semivowels, some are called liquids, because sometimes, set in one syllable after other consonants, they fail and are excluded from the metre. Of these two are liquid among the Latins, L and R, as in fragor, flatus. The rest, M and N, are liquid among the Greeks, as in Mnestheus. The old writing consisted of seventeen Latin letters. Hence they are called lawful (legitimae), by this reasoning: either because they begin with the vowel E and end in a mute sound, as the consonants do, or because they begin with their own sound and end in the vowel E, as the mutes do; and they are A B C D E F G I L M N O P R S T and U. The letter H was added later only for aspiration. Hence it is thought by many to be an aspiration, not a letter, which is called the mark of aspiration because it raises the voice. Aspiration is a sound uttered more fully, and its contrary is prosodia, a sound evenly inflected. The letter K was first added to the Latin letters by the schoolmaster Salvius, to make a difference in sound between the two letters C and Q; it is called superfluous because, except in Kalendae, it is judged superfluous; for we express everything by C. The letter Q neither the Greeks sound nor the Hebrews. Except for the Latins, no other language has it. It did not exist at first. Hence it too was called superfluous, because the ancients wrote everything with C. The letter X did not yet exist among the Latins up to the time of Augustus, and fittingly at this time, when the name of Christ became known, which is written by means of it, since it figures the sign of the cross; but in its place they wrote C and S, whence it is called double, because it stands for C and S, and so it has a name composed of the same letters. From the Greeks, however, Latinity borrowed two letters, Y and Z, on account of Greek names, and these were not written among the Romans until the time of Augustus, but for Z they put two S’s, as hilarissat; for Y they wrote I. To each letter three things belong: name, how it is called; figure, by what character it is marked; power, which is held to be a vowel and which a consonant. By some, order is added too, that is, which precedes and which follows, so that A is first, and B next. A is first of the letters among all nations because it opens the voice first for those being born. The nations gave the names of the letters from the sound of their own language, having noted and distinguished the sounds of the mouth. For after they observed them, they gave them both names and figures; and they formed the figures partly by choice, partly from the sound of the letters: for example I and O, of which the one, as its sound is slight, has a thin stroke, the other a full sound, as also a full figure. Nature gave the power; the will, the order and the apex. Between the figures of the letters and the apex the ancients distinguished: the apex is so called because it is far from the feet and is set on the top of the letter. It is a line lying evenly drawn above the letter. The figure is that by which the whole letter is written.