Vietnamese apex

In the 17th century, a curved diacritic with the Latin name of apex (used then for "tilde" on top of Portuguese vowels or in Latin abbreviations) was adopted to mark final nasalization in the early Vietnamese alphabet. It derived from the Portuguese tilde which often had that shape in the 17th century. The tilde used today, derived from Greek circumflex (perispomeni) with that shape, was also used to mark one of the tones. In his 1651 Dictionarium Annamiticum Lusitanum et Latinum, Alexandre de Rhodes describes the diacritic:

The apex appears atop $\langleo\rangle$, $\langleu\rangle$, and less commonly $\langleơ\rangle$. 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, an allophone of 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 $\langle-ng\rangle$ (representing ), 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 ao᷄ and ou᷄ 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.