Series System

As stated in the previous post, each consonant has one of the two inherent vowels depending on which series it is in--/ɑɑ/ for the first series and /ɔɔ/ for the second series. The term series was first referred to by Henderson (1952:151) as register, which is the distinction of pitch (first register has higher pitch than the second register). Later, Jacob (1968:4) had a different description whereby the first register is clear ('head' voice and a certain degree of tension), and the second register is breathy ('chest' voice and a comparatively relaxed utterance). However, Huffman (1978:2) suggests that Khmer may never have been a register language. He added that the term "register" should be reserved for a language in which there is a complete dichotomy of phonation type throughout the vowel system.

Then Huffman (1970:15) proposed the term "series" instead. Khmer orthographic consonants are divided into two series: first series and second series. Similarly, dependent vowels are also divided into two series, and each vowel symbol has two vowel qualities, /ɑɑ/ in the first series and /ɔɔ/ in the second series.