(Why) did the Thai script convert Sanskrit द /d/ to /th/ and then introduce its own character for /d/?

Question

The first section of the Thai alphabet/abugida seems to follow Sanskrit pretty closely, with just a couple of additions.

I believe that Sanskrit had the consonant /d/, which is represented by द in the Devanagari script and became ท in the Thai script, where it has the phonetic value /th/.

I'm struggling to understand why the phonetic value changed if (as I believe) Thai did have the consonant /d/.

Seemingly, an additional character, ด, was added, even though ท was already available (and the same can be said about ฎ and ฑ).

Is there any explanation for why the consonant ท was not simply given its original value? Is there known to have been some difference between this /d/ and the Thai /d/, or perhaps some practice of pronouncing Sanskrit words transliterated using that consonant with /th/, while home-grown words used /d/? Is it known whether ด pre-dates ท? Answers to any of these questions would be very welcome.

Answer 1

We have a case of historical spellings. We'll use the descendants of the alveolar stops as the easiest to understand:

• ISO Romanisation: ta - tha - da - dha
• Devanagari: त - थ - द - ध
• Khmer script: ត - ថ - ទ - ធ
• Thai script: ต - ถ - ท - ธ

When the Thai script was developed the late 13th century, there was already literacy in Pali and Mon-Khmer scripts. Hence, one reason for the design was backward compatibility. But this was not to some "original" or "authentic" Sanskrit pronunciation, but to the usual way that people had learnt to read these texts in Old Thai.

For the purposes of Old Thai loanwords from Sanskrit, Pali and Mon-Khmer, the four letters were more-or-less sufficient. At the time, Old Thai seemed to have merged द and ध into */d/, and so ท and ธ were both */d/. However, the issue of different phonologies was realised: Old Thai in the 13th century had another dental plosive, which has been theorised to be */ʔd/ pre-glottalised or perhaps */ɗ/ implosive.

There was a robust contrast between this phoneme and the other dental stops (unlike in late Khmer of the same period), so it appears that a new letter was created for it, ด, based on modifying ต. Although it may seem to us that using a version of /t/ to create a character */ʔd/ is counter-intuitive, one article points out that these pre-glottalised consonants "are considered to be voiceless because the glottal feature is voiceless". This is repeated for */ʔb/ บ created out of /p/ ป (Devanagari equivalent प). Slightly differently, it is theorised that */ʔj/ อ came from a presumed /ɦ/ ฮ (Devanagari equivalent ह), although it seems that there was already a functional role for it in the new script (placeholder consonant, which did not really exist as a role in Devanagari). Note that there is no */ʔg/ reconstructed for Old Thai, and it doesn't appear in the Thai script either!

Note that there are other add-ons of the Thai script in Old Thai aside from the pre-glottalised consonants; there are the extra fricatives (velar: ฃ and ฅ; labial: ฝ and ฟ; palatal [but really voiced alveolar]: ซ) which were just not a thing in medieval Indian languages. These were formed from the voiced unaspirated and voiced aspirated series.

However, as the "cataclysmic" sound change that swept through the tonal languages of China and Southeast Asia wore on between the 14th and 17th centuries, initial devoicing occurred, leading to a reorganisation of the consonant inventory: preglottalised consonants became voiced obstruents, and the voiced obstruent consonants became voiceless aspirated consonants in Central Thai (whereas in Northern Thai they became voiceless unaspirated).

This is best summarised diagrammatically as such:

Answer 2

I believe the full answer would take a whole book. Here's the brief:

ท ([d] → [tʰ]) is not alone here.

1. The original Pali consonant list had consonants ordered according to where they are produced in human's vocal tract
   (I believe this was a very unique feature of a writing system in the entire history of mankind);
2. During the course of time, the phonetic value of the consonants have been changed; the entire groups of consonants 'folded' into voiceless aspirates;
3. New symbols were introduced in order to restore the ancient pronunciation;

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The table below shows Pali consonants and its original phonetic values. I compiled it from several sources.

One may notice second letters in cells, created to restore the ancient pronunciation after the original consonant changed its phonetic value. For instance, as soon as บ shifted [p] → [b], the ป was introduced.

In some cases, it didn't work; see the sad story of ฟ, which was introduced after พ shifted [b] to [pʰ], with the obvious aim to have some letter to write [b], but then ฟ itself underwent another shift to [f].

This table also shows how consonants obtained its consonant class (which governs tones):

• Voiceless unaspirated became Mid-class (green on diagram);
• Voiceless aspirated became High-class (red);
• All Voiced consonants became Low-class (blue);

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I'm struggling to understand why the phonetic value changed if (as I believe) Thai did have the consonant /d/.

Richard Wordingham says in his article:

The second problem is the 'great consonant shift' whereby old voicing contrasts were lost in much of East Asia, covering most Tai, Mon-Khmer and Chinese dialects. (The change is not complete - some areas have escaped the change.)

Consequently, the more conservative Sinhalese and Burmese pronunciations are quite different to the Thai and Lao (and Mon and Khmer) pronunciations. The Thai and Lao pronunciations have replaced voiced stops by voiceless aspirates.

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Is there any explanation for why the consonant ท was not simply given its original value?

Quoting the same article:

The ideas of the new symbols it to restore the ancient pronunciation. Just as a Classical Latin pronunciation differs greatly from English legal Latin or Roman Catholic Church Latin, and is very different to how Latin loan words are pronounced in English, the modern Thai consonant sounds are very different to the ancient Pali sounds.

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[…] or perhaps some practice of pronouncing Sanskrit words transliterated using that consonant with /th/, while home-grown words used /d/?

Quoting the same article:

The problem with the traditional symbols is that they are pronounced quite differently in Thai. พุทฺธ is /buddʰa/ in the ancient pronunciation, but พุทธ is /pʰut-tʰa/ in Thai pronunciation. (Thai doesn't use PHINTHU.) An analogy is that 'Caesar' is pronounced /siːzə/ in British English, but is approximated as /kaɪsar/ in a Latin class in England.