◌᷃
This template inserts a character resembling an apex diacritic as found in Dictionarium Annamiticum Lusitanum et Latinum by Alexandre de Rhodes. This diacritic has not been included in the Unicode standard, so a lookalike, U+1DC3, is used instead.
The first parameter is the letter without the apex, that is, "o" or "u" with a horn and/or tone mark.
{{mkh-mvi-apex|o}} → o᷃{{mkh-mvi-apex|ơ}} → ơ᷃{{mkh-mvi-apex|ù}} → u᷃̀] → ſou᷃] → ſou᷃] → ſou᷃] → ſou᷃