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.

Subscripts

Huffman (1970:35):

"When two orthographic consonants are pronounced consecutively (without an intervening vowel) within a word, the second (and sometimes, in medial position, a third) consonant symbol is written in a special subscript form below the first symbol. [...] These subscripts are called ជើងអក្សរ /cəəŋ qaqsɑɑ/ 'consonant's feet' in Cambodian [...]."

The Unicode Consortium (2015:614) describes a Khmer subscript as a symbol representing any consonant following the first consonant in an orthographic syllable. The Unicode character used to represent this is called Khmer Sign Coeng ្ [U+17D2]. For instance, to get a Khmer subscript, one should key-in the Khmer Sign Coeng and a consonant right after it (្ + ង /ŋ/ -->  ្ង /ŋ/).
The table below shows a list of consonant and their corresponding subscript underneath each of them. Consonant ឡ does not have a subscript.

Khmer Subscripts

Orthographic Consonants

There are 33 orthographic consonants (Um and Seng 2012:5) in Khmer, and they are organized as shown in the table below. Students who study Khmer have to recite them in that exact order--counting from left to right and top to bottom. They are divided into six verses (or groups) according to their place of articulation--from the back to the front part of the mouth, in other words, velar to bilabial (Huffman 1970:13). The last verse is called an 'Odd' verse because consonants in this verse have different place of articulation. There are five approximants (យ /j/, រ /r/, ល /l/, វ /w/, ឡ /l/), two fricatives (ស /s/, ហ /h/), and a glottal stop (អ /ʔ/). Consonants with a gray background are in the second series which bear /ɔɔ/ as their inherent (embedded) vowel; others are in the first series which bear /ɑɑ/ as their inherent vowel. This means that when a consonant stands on its own, they are pronounced with their respective inherent vowel as long as there is no dependent vowel attached to them.

For example,

·         ក is pronounced as /kɑɑ/,

·         ខ is pronounced as /kʰɑɑ/,

·         គ is pronounced as /kɔɔ/,

·         ឃ is pronounced as /kʰɔɔ/,

·         ង is pronounced as /ŋɔɔ/, and so on.

Velar	ក /k/	ខ /kʰ/	គ /k/	ឃ  /kʰ/	ង /ŋ/
Palatal	ច /c/	ឆ /cʰ/	ជ /c/	ឈ /cʰ/	ញ /ɲ/
Alveolar[1]	ដ  /ɗ/	ឋ /tʰ/	ឌ  /ɗ/	ឍ /tʰ/	ណ /n/
Alveolar[2]	ត /t/	ថ /tʰ/	ទ /t/	ធ  /tʰ/	ន  /n/
Bilabial	ប  /ɓ/	ផ /pʰ/	ព  /p/	ភ /pʰ/	ម  /m/
Odd	យ /j/	រ /r/	ល /l/	វ /w/	ស /s/	ហ /h/	ឡ /l/	អ /ʔ/

Khmer-national scholars assume that the aspirated plosives are in fact a combination of the unaspirated plosive /p, t, c, k/ and the glottal fricative /h/ (Ieu 1967:42-43). In the table above, every second and forth orthographic consonants are originated from the combination of the first and the third orthographic consonants of each row and the glottal fricative /h/ (as shown in the table below).

The Aspirated Plosive Orthographic Consonants

---

[1] Huffman (1970:14) called these 'Retroflexes', but they are actually not retroflexes. There is no retroflexes in Khmer.

[2] Huffman (1970:14) called these dentals, but they actually are alveolar.