Note: By "Modern Thai" this article means Central Thai or Standard Thai.

Consonants

Thai consonants in writing are divided into "High," "Medium," and "Low" classes. This classification affects the tone each consonant carries. By categorizing each consonant’s initial sound and its class, we observe certain patterns: None of the consonant initials have all three class versions.

The following consonants have both high and low versions.

The following consonants only have low versions by themselves. However, they can be modified to form high-class sounds when written as digraphs with ห (Ho Hip).

Additionally, อย is classified as a Mid consonant.

The following consonants are exclusively mid-class.

The division of Thai consonants into High, Medium, and Low tone classes becomes clearer when we examine their historical pronunciation, which combined elements of Sukhothai and Sanskrit phonetics. Here’s a breakdown of the consonant sounds based on their articulation categories in Sukhothai and their original Pali or Sanskrit pronunciation:

Both voiced stops have become aspirated stops in modern Thai. This transformation is the origin of the high and low tone distinction in many consonants.

Additionally, some consonants from Pali and Sanskrit merged in Sukhothai Thai:

• In Sukhothai, no clear distinction was made between voiced and breathy stops; for instance, both ฆ and ค were pronounced as /g/.
• Retroflex and dental consonants merged, which explains why modern Thai has four low "th" sounds, originating from /ɖ, ɖʱ, d, dʱ/.
• The three fricatives /ɕ, ʂ, and s/ merged into a single /s/ sound in modern Thai.

Some consonants appear more frequently than others in modern Thai, with certain historical sounds like /x/ and /ɣ/ disappearing entirely. Using a corpus such as this one, we can determine the frequency of each consonant in Thai script. Here are the frequencies for initials that have multiple characters:

ญ and ย are sometimes considered distinct due to ญ carrying an /n/ sound in the final position.

Vowels

Thai has nine vowels, each with a long and short version.

In open syllables, the inherent vowel is /a/, while in closed syllables, it is /o/.

To represent /a/ in closed syllables, ◌ั (Mai Han Akat) is used. Conversely, the structure โ◌ะ is used to write /o/ in open syllables.

Certain vowels, like /e/, /ɛ/, and /ɔ/, are inherently long. The ะ (Nom Nang) marker is needed to "close" the syllable, often implying a glottal stop. In closed syllables, ◌็ (Mai Taikhu) is used to shorten these inherently long vowels.

The symbol ◌อะ is not used for short /ɔ/ in open syllables; instead, เ◌าะ represents short /ɔ/, and เ◌า without Nom Nang signifies the sound /aw/.

The following table summarizes the main diphthongs:

As with monophthongs, Nom Nang is used to shorten diphthongs. Historically, ใ◌ represented /aɯ/ but has since shifted to /aj/.

The combination ◌ัว is not used for the sound "aw" because it would read as "-wa" in Thai.

Tone Split

The Proto-Tai language originally had three phonemic tones, with an additional "Tone D" that sometimes appears as a fourth tone. However, Tone D only occurs in "dead" syllables, those that end with an obstruent sound (e.g., stops or fricatives).

The Thai script includes four tone markers, but only the first two markers Mai Ek and Mai Tho, are commonly used.

Originally, Mai Ek and Mai Tho corresponded to "Low" and "High" tones, respectively. Over time, sound changes led to a tonal split based on the consonant class (Low, Mid, or High) and syllable type. This resulted in central Thai’s five tone system.

Live syllables (those with open or sonorant final sounds) exhibit a tonal shift based on the original Proto-Tai tone and the consonant class:

• Original Low Tone: Mid and High consonants retain a low tone, while Low consonants shift to falling.
• Original Mid Tone: Low and Mid consonants remain mid-tone, while High consonants become rising.
• Original High Tone: Low consonants remain high, while Mid and High consonants shift to falling.

This structure lets us rerpesent of the five tones in live syllables by using the correct combination of consonant class and tone marker:

We can see how Mai Tri and Mai Chattawa are used with mid-class consonants to create rising and high tones, as they lack these tones otherwise.

In dead syllables (those with obstruent endings), the tone split is simpler. There is no tonal distinction between Mid and High consonants, leading to only three possible tones in dead syllables:

Thus, dead syllables have only three tones:

In summary, the combination of consonant class, tone marker, and syllable type allows Thai to maintain a rich tonal system that emerged from the simpler Proto-Tai tones.