Vietnamese tilde
The Vietnamese tilde, also known by its Latin name of apex, was a curved diacritic used in the 17th century to mark final nasalization in the early Vietnamese alphabet. It was an adoption of the Portuguese tilde, and should not be confused with the tone mark ngã, which is encoded as a tilde in Unicode (and in Vietnamese derivatives of ISO-8859-1 such as VISCII, VPS or Windows-1258), despite actually being an adoption of the Greek perispomeni. Apex is the name used in contemporary Latin texts.
In his 1651 Dictionarium Annamiticum Lusitanum et Latinum, Alexandre de Rhodes describes the diacritic:
The apex appears atop ⟨o⟩, ⟨u⟩, and less commonly ⟨ơ⟩. As with other accent marks, a tone mark can appear atop the apex.
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. The apex apparently fell out of use during the mid-18th century, being unified with ⟨-ng⟩ (representing /ŋ/), in a major simplification of the orthography, though the Vietnamese Jesuit Philipphê Bỉnh (Philiphê do Rosario) continued to use the old orthography into the early 19th century. In Pierre Pigneau de Behaine and Jean-Louis Taberd's 1838 Dictionarium Anamitico-Latinum, the words ao᷃ and ou᷃ became ong and ông, respectively.
The Middle Vietnamese apex is known as dấu sóng or dấu lưỡi câu in modern Vietnamese. 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.
Examples
Obtained from Dictionarium Annamiticum Lusitanum et Latinum, a trilingual Vietnamese, Portuguese and Latin dictionary by Jesuit Alexandre de Rhodes.
The entry for dĕóu᷃ shows distinct breves (ĕ), acutes (ó), and apices (u᷃).
The entry for blu᷃́ shows that a vowel with an apex can take on an additional tone mark, in this case an acute.
The entry for bao giơ᷃ đến illustrates the difference between a horn and an apex.
