In his 1651 Dictionarium Annamiticum Lusitanum et Latinum, Alexandre de Rhodes describes the diacritic:[5][6][7] The third sign, finally, is the apex, which in this language is entirely necessary because of a difference in the ending [i.e. of a word], which the apex makes entirely distinct from the ending that m or n makes, with a meaning entirely diverse in words in which it is employed.
[9] According to canon law historian Roland Jacques, the apex indicated a final labial-velar nasal [ŋ͡m], an allophone of /ŋ/ that is peculiar to the Hanoi dialect to the present day.
[10] In Pierre Pigneau de Behaine and Jean-Louis Taberd's 1838 Dictionarium Anamitico-Latinum,[11] the words ao᷃ and ou᷃ became ong and ông, respectively.
The apex is often mistaken for a tilde in modern reproductions of early Vietnamese writing, such as in Phạm Thế Ngũ's Việt Nam văn học sử.
[12][13] Obtained from Dictionarium Annamiticum Lusitanum et Latinum, a trilingual Vietnamese, Portuguese and Latin dictionary by Jesuit Alexandre de Rhodes.