EDIT Ssa This letter belongs to the consonants spoken in the first register. The correct transliteration therefore is ssa, not sso.
This letter is only used for Pali and Sanskrit Transliteration.

The first register are the sounds spoken in a normal (head) voice quality with a pretty high pitch. The second register uses a deeper and breathy kind of voice with a lower pitch.

The same goes for U+178E Khmer Letter Nno, which is actually pronounced nna.

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