Etymologiae, Book I, chapters 18-19: De accentibus to De figuris accentuum
Chapter XVIII. On accents The accent, which is called in Greek prosodia, took its name from the Greek. For in Greek pros is in Latin “to,” and ōdē in Greek is in Latin “song.” For this name has been rendered word for word. The Latins, however, have other names too. For they say accentus and tonos and tenores, because there the sound rises and ceases. The accent is so called because it is next to song (iuxta cantum), as the adverb is so called because it is next to the verb. The acute accent is so called because it sharpens and raises the syllable, the grave because it presses down and lowers it. For it is the contrary of the acute. The circumflex, because it is made of the acute and the grave. For beginning from the acute it ends in the grave, and so, while it rises and falls, a circumflex is made. The acute and the circumflex are alike. For both raise the syllable. The grave seems contrary to both. For it always presses the syllables down, while those raise them, as in: “Whence the Titan comes, and there night buries the stars.” Here unde is grave. For it sounds less than the acute and the circumflex. A monosyllable, if it is naturally short, as vir, or long by position, as ars, will have the acute accent. If, however, it is naturally long, as res, the circumflex. A disyllabic word, if it has the first syllable naturally long and the last short, is circumflexed, as Musa; otherwise it is acuted. A trisyllabic word, if it has the middle syllable short, as tibia, then we put the acute on the first. If, however, it has the second naturally long and the last short, as Metellus, then we circumflex the middle. Words of four and of five syllables are governed by the rule of trisyllables. The grave accent can be placed with one other accent in a single word, but never with both; as Catullus. In a compound word there is one accent. Accents were found either for the sake of distinction, as in “on the green shore a sow is seen,” lest you say ursus; or for pronunciation, lest you say meta briefly and not lengthened; or for the sake of discerning ambiguity, as ergo. For when the go is lengthened it signifies “cause”; when shortened, a conjunction.
Chapter XIX. On the figures of the accents The figures of the accents are ten, which the grammarians set down for the distinctions of words. Oxeia, that is, the acute accent, a line drawn from the left side upward to the right. Bareia, that is, the grave, a line brought down from the top at the left to the right. Perispomenē, that is, the circumflex, made of the acute and the grave, is expressed thus. Makros, that is, long, is a stroke lying flat.
Brachys, that is, short, is the lower part of a circle, lying thus. Hyphen, that is, the joining, because it connects two words, a stroke subjoined beneath the line, curved. Diastolē, that is, distinction, which on the contrary separates, the right part of a circle placed under the line. The apostrophe is likewise the right part of a circle, set at the top of the letter, by which mark it is shown that the final vowel is wanting in the word, as tribunal for tribunale. Dasia, which is interpreted as aspiration, that is, where the letter H ought to be placed, is marked with such a figure. Psilē, which is interpreted as dryness, or pure, that is, where the letter H ought not to be, is shown with such a mark. The Latins made the figure of these two accents from the very letter of aspiration. Hence if you join these, you have made the same mark of aspiration. Again, if you split its middle stroke, you make dasia and psilē.