The grave accent indicates that an open-mid or open vowel is pronounced stressed. Stressed close-mid or close vowels are indicated with an acute accent ´.
Institutiones linguae latinae et graecae pro infima grammatices ad normam Emmanuelis Alvari et Jacobi Gretseri Societatis Jesu, in usum scholarum Provinciae S. J. ad Rhenum superiorem nova methodo adornatae. Editio quarta, Augusta Vindelicorum, 1779, p. 212 in Erster Anhang. De orthographia.:
(`) (´) Accentus gravis & acútus. Gravis (`) wird zum Besten der Lernenden nicht unrecht gebraucht bey den Adverbiis, um sie von anderen Partibus Orationis zu unterscheiden, als: Eò, quò, tantò, doctè, &c. [...] ( Nota. Wie die Interpunctiones recht zu gebrauchen seyen, wird in der Lehr de Periŏdis erörtert.
Thomae Ruddimanni institutiones grammaticae latinae. Curante Godofredo Stallbaum. Pars secunda syntaxin continens, Lipsia, 1823, p. 39 of the Appendix. Grammaticae latinae institutionum pars tertia ex compendio Ruddimanni repetita:
Toni sive Accentus sunt tres, Acutus, Gravis, et Circumflexus. [...] Gravis est qui syllabam gravat, seu deprimit; ac signatur lineola obliqua a sinistra in dextram ascendente, hoc modo [`]: ut, doctè. [...]
Allen Fisk, Adam's Latin Grammar; simplified, by Means of an Introduction: Designed to facilitate the Study of Latin Grammar, [...]. Fifth Edition, from the second Edition, revised and corrected, New-York, 1830, p. 182:
"There are three accents [...] 2. The grave or base accent depresses the voice, or keeps it in its natural tone; and is thus marked [`]; as, doctè. This accent properly belongs to all syllables which have no other accent. [...] The accents are [..] seldom marked in Latin books, unless for the sake of distinction, as in these adverbs, aliquò, continuò, doctè, unà, &c. to distinguish them from certain cases of adjectives, which are spelt in the same way. [...]
(лингвистика) A diacritical mark, both in the Cyrillic and Latin script, used to denote a short-rising accent. Not used in everyday writing. Can be used on vowels and the syllabic R:
In Vietnamese handwriting and signmaking, this tone mark may be written as a horizontal line, like a macron (which does not exist in Vietnamese), and the letter I/i retains its tittle.
In earlier versions of Unicode, ̀ was used to represent this tone mark.