At least the Middle and High classes have to be learned - consonants that are not contained therein are automatically placed low class.
The three consonant classes
[ak\ sa:wn\/ gla:ng--]
|ก จ ฎ ฏ ด ต บ ป อ
[ak\ sa:wn\/ su:ng\/]
|ข ฃ ฉ ฐ ถ ผ ฝ ศ ษ ส ห
[ak\ sa:wn\/ tam\]
|ค ฅ ฆ ง ช ซ ฌ ญ ฑ ฒ ณ ท ธ น พ ฟ ภ ม ย ร ล ว ฬ ฮ
The belonging of a consonant to one of the three classes together with other criteria determines the tone pitch of a syllable. The rules for this are not entirely trivial and will be presented in detail here.
The third class (Low Consonants) contains 14 consonants with sounds already available in the second class (High Consonants).
Same sounds of consonants in second and third class
|ศ ษ ส
|ค ฅ ฆ
|ฑ ฒ ท ธ
These 14 consonants of the third class are called Paired Consonants because they have a counterpart.
But there are ten more consonants with sounds only available in the third class. They are called Unpaired Consonants.
Subdivision of the consonants of the low class
|3.1 Paired Consonants
[ak\ sa:wn\/ khu:/\]
|ค ฅ ฆ ช ฌ ฑ ฒ ท ธ พ ภ ฟ ซ ฮ
|3.2 Unpaired Consonants
[ak\ sa:wn\/ di:au\]
|ง ญ ณ น ม ย ร ล ฬ ว
Why is this important?
According to the tone rules with low initial consonants can be formed no syllables with low or rising tone. This is not a problem in the sounds of paired consonants, because there are counterparts in the second class which can be used instead.
For the unpaired consonants there is no such alternative. But since there were many words that require a deep or a rising tone on syllables with the sound of an unpaired consonant, there is a workaround available. As you can learn in the article about double consonants (IV.), there is a way to lend the tone rules from another class to the unpaired consonants of the third class.