Letters expletive, and marks on letters.
In French there are two letters expletive —l and t. The former, called `l ephelcystic,' is placed before on if the preceding word ends with a vowel, as si-l-on. The latter is called “t euphonistic,” and is used in interrogative sentences between the third person singular of verbs ending with a vowel, and a pronoun beginning with a vowel, as gelle-t-il? a-t-elle?
The chief accents are the grave (), acute () and circumflex .
Two dots over the latter of two vowels (called diocresis, signify that each vowel is to be sounded, as Aët'ius (4 syl.).
A hyphen between two or more nouns or syllables denotes that they form a compound word, as mother-in-law. The hyphen in French is called a “trait d'union,” as irai-je.
In French, the mark (,) under the letter c is called a cedilla, and signifies that the c (which would otherwise be = k) is to be pronounced like s, as ca (sah), and garcon (garson).
A small comma (`) over an, a, o, or u, in Scandinavian languages, is called an umlau, and a vowel so marked is called an umlaute (3 syl.).
(or ) over the vowel o in German is called a zweipunct (2 syl.), and gives the vowel the sound of a French eu, as in peu, etc.; but over the vowel u it gives it the sound of the French u in dût.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894