There are 9 middle-class consonant symbols: จ (j), อ ( ), ก (gk), บ (b), ป (bp), ด (d), ต (dt), ฎ (d), ฏ (dt). We haven’t met the last two yet; they are rare and we mention them here for completeness.
The class of a consonant symbol determines which set of rules to apply to give the tone of the vowel sound that follows. The middle-class consonants are the best behaved of the three consonant classes (low and high are the other two) in the sense that the rules are the simplest for them.
All the consonant symbols we’ve looked at so far have been low or middle class. With two exceptions, บาท (bàat) and น้ำ (náam), all of the words mentioned so far have no tone (or have the middle tone, if you prefer): they are said at the normal pitch of one’s voice and there is no change in pitch. So what do all these words have in common? The answer is that all the syllables in these words have ‘live’ endings and no tone markings.
A live ending is an extendable one in the sense that it is possible to elongate the final sound. In Thai there are 3 live consonant endings: m, n, ng. (The remaining possible consonant endings in Thai, namely, gk, bp, dt, are ‘dead’ – the sounds cannot be extended.) The live vowel endings are any long vowel sound (such as า, อ, ี) or any combination of vowel sounds, technically known as diphthongs or triphthongs, which don’t end in a very short ‘a’ sound (such as ไ (ai), ัว (ua), อย (oy)).
So the rule is, a live-ending syllable with no tone markings and starting with a middle-class consonant symbol has no tone. This is also true for low-class consonant symbols if the consonant symbol is not immediately preceded by a high-class consonant symbol.
So how can we remember whether a consonant symbol is middle class or not? If the corresponding sound is j, g, b, bp, d, dt, or it is the zero consonant อ then it is middle class.
Thai Writing 