A Latin tutorial said "The first letter of a sentence in Latin is not capitalized." That's strange. And most Latin texts I have seen do not obey that rule. Most Latin tutorials I have learned did not mention this rule. Until now I have found only two examples where the first letter is in lowercase. The first example is the texts in that tutorial. The second is New Latin Grammar Every example sentence in this book starts with a lowercase letter.
So, how did the rule appear?
It should be remembered that the ancient texts did not have letter case as we know it. Lower case is a late antique/medieval manuscript development. You did have some "lower case" letters in antiquity, but these weren't defined by any rules. Rather, it was an organic process in which letter shapes were rounded for speed in writing, which is the actual origin of lower case letters to begin with.
But because text after text of capital letters (with no or minimal word division) is not conducive to easy reading, modern editors adopt modern standards. What those standards are will change from publisher to publisher and from editor to editor.
So with modern texts, there isn't really a "rule" per se; it's merely convention. Some texts indeed do not capitalize the beginning of each sentence. Some do. Grabbing a bunch of texts (Loebs and OCTs which I have handy) off the shelf, you'll see that there isn't a hard and fast rule. Of the three that did not capitalize after the period, two are of poetry, one is of prose, all originally published in the 20s and 30s. The four that did capitalize are all of prose, but are contemporary (1910s to late 1930s) with the other three.
Mynors' Vergil does not capitalize after every period, but does at the beginning of new sections. Owen's Perseus and Juvenal doesn't even do that. The Livy OCT capitalizes after some periods, but not all. Watt's OCT of Cicero's letters capitalizes after every period, but uses semicolons frequently. Pontet's Caesar is the same.
The tutorial is thus not really honest when it gives this as a "rule," unless it means it as an explanation for its own adopted conventions.
There is no such rule. It's a convention that some publishers follow (to varying degrees) but others don't, as a matter of editorial policy.
For example, I believe that Teubner has traditionally capitalized the first word of each paragraph/section in its Latin texts, but otherwise doesn't capitalize the first word of a sentence unless that word is a proper noun.
By contrast, in all but one of the Oxford texts that I pulled off my shelves, the first word of every sentence is capitalized. (The exception was Ogilvie's 'Oxford red' of Tacitus's Agricola.) I looked only at prose works, though.
Cambridge seem to leave the matter up to the individual editor. Some of the Cambridge texts on my shelves capitalize the first word of every sentence, others capitalize only the first word of every paragraph/section, and others don't capitalize the first word of any sentence (unless the word is a proper noun).
For what it's worth, I'll note that typographer, book designer, and poet Robert Bringhurst, in The elements of typographic style – a work that has sometimes (and not without reason) been criticized for being overly dogmatic and even impractical - says the following in his discussion of spaces between sentence (p 29–30 in the 4.0 version of the book):
As a general rule, no more than a single space is required after a period, a colon or any other mark of punctuation. Larger spaces (e.g., en spaces) are themselves punctuation.
The rule is sometimes altered, however, when setting classical Latin and Greek, romanized Sanskrit, phonetics or other kinds of texts in which sentences begin with lowercase letters. In the absence of a capital, a full en space (M/2) between sentences may be welcome.
So Bringhurst, speaking as a typographer, seems to take it for granted that the sentences in a Latin text will begin with lowercase letters. Nevertheless, given that most editions of Latin prose (Oxford, Teubner, Cambridge, Loeb, and others) are printed with justified text blocks, where the spacing between words and sentences is highly flexible to achieve the justification, his point about extra spacing doesn't really apply.
Latin texts don't really differenciate between upper and lower case. In Ancient times, tables and engravings used upper case. For stylus work on wax tablets and papyrus the romans often used "cursive" letters.
Medieval Latin script is... something. You have Trajan script, Uncial Visigothic, Carolingian, Beneventan etc. etc. that all wrote their letters a bit different, depending on what material and with what they wrote. (Visigothic use what we recognize as lowercase letters, Trajan and Uncial used Capitals).
The Carolingian script used both Upper- and lowercase script and was the first one to do so. This went on for years, through the Renaissance and the printing press. During these times lowercase became standard and capitals were more of a thing of emphasis. (See "We The People" on the DEclaration of Independence as an example).
Rules of capitalization in western-european languages came about in the late 18th to early 19th century when Grammarists overhauled much of our written languages, but never established a single convention for Latin.
So, with this long history lesson done: The answer you're looking for is that your book is wrong, but also right, depending on what time period and script we're talking about. Write it in whatever convention you like or is required of you. Just please don't use Carolingian script and stick to the one we use nowadays ;)
In the school-text: "So You Really Want to Learn Latin" (vols. I-III)/ Nicholas Oulton, all the Latin sentences begin with lower-case letters (unless a proper noun is involved). Also by this publisher, Galore Park, "Latin Translations"/ Betty Halifax, the same convention.
With a standard above Oulton's vol. III; but, still a school text: "Latin Prose Composition"/ North & Hillard (Answer Key), the Latin sentences begin with higher-case letters.
Long before the time of LatinStack, I e-mailed Mr. Oulton, through the publisher, on this point. He confirmed that Latin sentences begin with lower-case letters.
As a new student, despite the instinctive discomfort of beginning sentences with lower-case, I accepted this. Over the last five-years, in all my postings, which have included Latin, I have started sentences with lower-case. Nobody has ever questioned this. Interesting, because experience indicates that here (possible) mistakes are jumped-on very quickly--which is as it should be.
If there is any doubt, if for example, you are sitting an exam., define what you are doing: by this-or-that convention/ text-book, I am using higher/ lower case....
