Unlike the order of the English alphabet, which has no underpinning logic, the order of Thai consonants is highly logical, though that may not immediately be apparent to the learner. In fact, the order is ultimately based upon the traditional ordering of Sanskrit. When we arrange the consonants according to this we get the following table:

Read this table starting with Group I, reading left to right (i.e. ก to ง). Repeat with groups II, III, IV and V.

Next read column f from top to bottom (i.e. ย to ว), followed by columns g and h.

So there you have all the consonants in order from ก to ฮ - from chicken to owl.

Now why are some cells split? First a little history: Thai script was devised based upon an Ancient Khmer script, which in turn was based upon an even older Indian script, Pallava. Pallava only has 33 consonants, as did Ancient Khmer, so it was necessary to invent new characters to represent sounds which could not be mapped to Pallava/Khmer. The new characters were made as variants of existing ones. So, for example บ, which corresponds to ប in Khmer, had one of its "arms" elongated to form ป. ซ is a variant of ช, created by adding a notch.

Groups I to V are based upon where in the mouth the sounds are made in Sanskrit - starting with Group I, where the sounds are made at the back of the mouth, then moving forward in the mouth, to group V, where the sounds are made with the lips. The following table summarises these groups:

If we highlight the irregular consonants (i.e. the consonants that were introduced to allow Thai script to represent characters from Indic languages which are not present in Thai) we get the following:

We can see that Group III is all irregular dentals, whilst Group IV is regular dentals.

We can also see that column d is all irregular, too. We'll return to that in a moment.

Now let's colour the consonants by class. Mid class is green, high class is red, and low class is blue.

We can now immediately see the meanings of most of the columns:

If we look at column f (the semi-vowels) we can see that they relate to the groups II to V. For example, ย is a palatal consonant, whilst ว is a labial one.

For column g, the high class s's, the group relationship doesn't apply since all three are pronounced the same.

Finally, in column g (the miscellaneous group), only ห has a corresponding Sanskrit character. I suppose that อ was created to represent a glottal stop, so words beginning ʔo such as องก์ and อก could be written. ฮ was created to allow loan words from English and from Chinese languages to be represented. (Since ห is a high class consonant it can't be used to represent a mid tone.) According to J. Marvin Brown, ฬ was invented to represent l in words borrowed from an "abherrant dialect of Sanskrit".

We can now see what happens when Indic loan words are transcribed into Thai, and also what happens to the pronunciation.

Group III in Sanskrit is not dental, but retroflex, i.e. the sound is formed with the tongue tip curled back. In other words, we have the transformation: retroflex (Indic) ➜ (Thai). So, for example, ṭ ➜ t, ṭh ➜ th, &c..

Turning now to the colums:

Notes:

1. Aspiration is the strong burst of breath that accompanies the release (end) of a consonant. So, for example, in Thai ท and พ are aspirated, and ต and ป are not.
2. Voiced consonants use the vocal chords to produce the sound; unvoiced ones do not involve the vocal chords. So, for example, ด is voiced, whilst ต and ท are unvoiced.
3. Plain consonants are neither voiced, nor aspirated
4. Nasal consonants are those pronounced through the nose. (The mouth is blocked.)