Etymologiae, Book I, chapters 25-27: De notis litterarum to De orthographia
Chapter XXV. On the marks of letters The ancients also made marks of letters among themselves, so that whatever they wished to signify secretly to one another through writings they might write by exchange. Brutus is a witness, who noted in these letters what he was about to do, while others did not know what these letters meant. Caesar Augustus also, to his son: “Since,” he says, “countless things happen constantly which it is fitting should be written by either and be secret, let us, if you wish, have such marks between us that, when something is to be written in marks, we write for each letter the following one, in this way: for A, B; for B, C; and so on by the same reckoning for the rest; for the letter Z, one must return to a double A.” Some also write with the words reversed.
Chapter XXVI. On the marks of the fingers There are also certain marks of the fingers, and of the eyes, by which people converse silently with themselves and at a distance. So it is a military custom that whenever the army agrees, since it cannot by voice, it pledges by the hand. Others, since they cannot by voice, salute by the movement of swords. Ennius, of a certain shameless woman:
“As if playing ball in a circle, she gives herself in turn and makes herself common. One man she holds, to another she nods, elsewhere her hand is busy, another’s foot she plucks, to another she gives a ring to wait for, with her lips she calls on another, with another she sings; yet to others she gives letters with her finger.” And Solomon: “He winketh with the eye, he stampeth with the foot, he speaketh with the finger.”
Chapter XXVII. On orthography Orthography is interpreted in Greek, in Latin, as right writing. For orthos means right, graphia writing. This discipline teaches how we ought to write. For as the art treats of the declension of the parts, so orthography treats of the skill of writing, for example, ad, when it is a preposition, takes the letter D; when it is a conjunction, the letter T. Haud, when it is an adverb of denying, is ended in D and aspirated at the beginning; when it is a disjunctive conjunction, it is written with the letter T without aspiration. Apud, the preposition, is written with D, as ad patrem, since the ancients often used apud for ad, two of the middle letters being taken away. Sometimes other letters are rightly put in place of other letters. Between the letters B and P there is a certain kinship. For instead of Burrus we say Pyrrhus. The letters C and G have a certain kinship. For when we say centum and trecentos, afterwards we say quadringentos, putting G for C. Likewise between C and Q there is kinship. For huiusce we write with C, cuiusque with Q. Cum, the preposition, is to be written with C; if it is the adverb, with Q. For we say quum lego. Deus is to be written with E alone; daemon is to be marked with the diphthong AE. Equus, which is the animal, is to be written with E alone. Aequus, which is just, is to be written with the diphthong AE. Exsul ought to be written with an added S, because an exile is one who is outside the soil (extra solum). Exultat is better written without the letter S. For since X itself consists of C and S, how, when it is in it, is another added to it again? Aequor is to be written with the diphthong, because the name is made from water (aqua). Forsitan is to be written with N at the end, because its full form is si forte tandem. Fedus, which is foul, is to be written with E alone; foedus, which is a pact, is to be written with O and the diphthong E. Formosus is written without N, because it is named from beauty (forma). Or perhaps from formus, that is, warm; for the heat of the blood produces beauty. Gnatus, which is son, is to be written with G, because it makes generatus. H, which is the letter of aspiration, in Latin is joined only to vowels: as honor, homo, humus, humilitas. It is aspirated also with consonants, but in Greek and Hebrew names. The interjections heus and heu are to be written with H. The letter I set between two vowels some thought should be written twice, as in Troia and Maia. Reason does not permit this. For three vowels are never written in one syllable. But the letter I set between two vowels is counted as double. Id, the pronoun of the neuter gender, is written with D, from is, ea, id, because it makes idem. But if it is a verb of the third person, it will be marked with T, from eo, is, it, because it makes itur. The ancients put the letter K before A whenever A followed, as kaput, kanna, kalamus. Now, however, Karthago and kalendae alone are written with it. All Greek names, whatever vowel follows, are to be written with K. Laetus is written with the diphthong, because laetitia is named from breadth (latitudo), the contrary of which is sadness, which causes narrowness. We sometimes use the letter L for the letter D, as latum for datum and calamitatem for cadamitatem; for calamity took its name from falling (cadendo). Whether maxumus or maximus, and how any like words ought to be written, was asked. Varro relates that Caesar was accustomed to pronounce and write such words with I. Hence, on the authority of so great a man, the custom was made that maximus, optimus, pessimus be written so. Malo is to be written with one L, because it is magis volo. Malle with two LL, because it is magis velle. Nolo also with one L; and nolle with two. For nolo is nevolo; nolle, nevelle. Os, if it signifies the face or a bone, is to be written with O alone; if a person, H is to be put before it. Ora of boundaries is written with O; hora of days with H. Onus, if it comes from burden, is written with O alone; if from honour, with the aspiration H. Praepositio and praeterea are to be written with the diphthong. Pene, which is a conjunction, with E; poena, which is punishment, with OE. The letter Q is rightly placed when the letter U follows it at once, and any other one or more vowels are joined, so that one syllable is made. The rest are written with C. Quae, the pronoun, is to be written with A; que, the conjunction, without A. Quid is written with the letter D when it is a pronoun; with T when a verb: whose first position is queo, quis, quit, and in composition nequeo, nequis, nequit. Quod, when it is a pronoun, is to be written with D; when a number, with T, because totidem is written with T. Quotidie is to be written with Q, not with C, so that it is quot diebus. The letter R has a communion with the letter S. So among the ancients it was said honos, labos, arbos; now honor, labor, arbor. Sat ought to be written with T, because its full form is satis. Sed ought to be written with D. For among the ancients sed was said sedum; we have cut off the two final letters. Tamtus, like quamtus, had M in the middle. For quam and tam, whence also quamtitas, quamtus, tamtus. Vae, the interjection, is to be written with A; ve, the conjunction, without A. Xps, because it is Greek, is to be written with X. So also Xrisma. Only Greek names are written with the letters Y and Z. For although the letter Z expresses the sound of iustitia, yet because it is Latin it is to be written with T. So militia, malitia, nequitia and other like words. In doubtful words also it was the custom of the ancients that, when the same letter had one meaning shortened and another lengthened, they put the apex of the long syllable; for example, whether populus signified a tree or a multitude of men was distinguished by the apex. So also where consonant letters were doubled, they put a sicilicus above, as cella, serra (planks). For the ancients did not double letters, but put sicilici above; by this mark the reader was warned that the letter was to be doubled.