Series Shifters

Some phonemes have two orthographic consonants such as: /k/ for ក and គ, /kʰ/
for ខ and ឃ, /c/ for ច and ជ, /cʰ/ for ឆ and ឈ and so on; but some others do not
have a series counterpart. That is when series shifters are needed. Series shifters are
orthographic diacritics which change the series of the consonant they are attached
to. They could change the series of consonants from the first series to the second or
vice versa. MUUSIKATOAN (៉), a diacritic which changes the series of 6 consonants
(យ, រ, វ, ង, ញ, ម) from the second series to the first series; and TRIISAP (៊), and
conversely changes the series of four consonants (ស, ហ, ប, អ) from the first series
to the second series. Tables below show 44 consonants in both first and second
series including the orthographic consonants which are modified by the series
shifters. The phoneme equivalent to each consonant symbol is given. 

Consonants in Their Respective Series (a)

Consonants in Their Respective Series (b)

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

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[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.