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Phonation and tone

Khmer once had a phonation distinction in its vowels, but this now survives only in the most archaic dialect (Western Khmer). The distinction arose historically when vowels after Old Khmer voiced consonants became breathy voiced and diphthongized; for example *kaa, *ɡaa became *kaa, *ɡe̤a. When consonant voicing was lost, the distinction was maintained by the vowel (*kaa, *ke̤a); later the phonation disappeared as well (,). These processes explain the origin of what are now called a-series and o-series consonants in the Khmer script.

Although most Cambodian dialects are not tonal, the colloquial Phnom Penh dialect has developed a tonal contrast (level versus peaking tone) as a by-product of the elision of /r/.