We are having a discussion on the length of the Thai vowels /e/, /e:/
and /E/ and /E:/, with particular regard to how they are represented
in the orthography, at http://www.thai-
ubb=get_topic;f=21;t=000016;p=1#000012 . In Thai spelling, the
difference between the short and long vowels is marked by a shortness
sign, but this is not allowed if a tone mark is required.

The history of /e/ and /e:/ seems simple enough - only /e/ is
inherited, /e:/ chiefly comes from loans from non-tone languages
(Sanskrit and Pali in large measure). Words with /e:/ should
therefore be expected to be placed in Tai [sic] tone class A or D,
the two pairs of tones writen without tone marks. For reading, this
gives the rule that a tone mark implies a short vowel.

For /E/ and /E:/, the general rule is that /E:/ is inherited. I am
not aware of any major source of loanwords with these vowels - they,
like a tone mark, preclude immediate derivation from Sanskrit or
Pali. (There are a few counter-examples if the word 'immediate' is
omitted.) However, there appears to be a rule that /E:/ is shortened
in words with Tai tone class B, i.e. with the pair of tones indicated
by a superscript vertical stroke, called 'mai ek'. Does anyone here
know the origin of this shortening, or even how consistent it is.

Are there any current sound changes significantly affecting these
vowels? There appear to be very discrepant descriptions of the
vowels of individual words. We do have sound clips, from at least
three different informants.

To confuse matters, Thai has many apparently sporadic lengthenings
and shortenings, not all of which are represented in the spelling.