ฃ. ฃวด

(Kho Khuat) is the third letter of Thai alphabets. It is a high sound consonant in the Thai tripartite consonant system. The letter is obsolete, being replaced universally by kho khai (ข. ไข่). There are currently no words using kho khuat in Thai language according to the Royal Institute Dictionary of 1999, the official standard current dictionary of the Thai language. However, kho khuat still has an entry in most dictionaries stating that it is obsolete, and is included on alphabet charts to preserve the traditional count of 44 Thai consonants. There is a minor movement for conservative spelling among some Thai publishers, one goal of which is to bring kho khuat and kho khon back into currency.

ฅ. ฅน

(Kho Khon) One such publisher is Butterfly Book House (สำนักพิมพ์ผีเสื้อ), which publishes children's literature both by Thai authors and Thai translations of foreign authors, such as Roald Dahl. In its books, words like ขวด and คน are spelled ฃวด and ฅน, despite the fact that the apparently "conservative" spelling is sometimes arguably not the etymologically correct one, as with ฅน for คน. As for the latter, a 2006 film set in 1890's Siam and titled in Thai: ฅนไฟบิน Flying Fire Person (in English: Dynamite Warrior), using kho khon (ฅ. ฅน) to spell ฅน Person. Person is the standard example word for "ฅ" in consonantal charts, but is spelled คน (Courtesy great explanation of Wikipedia.

The rule of using ฤ ฤา ฦ ฦา
2.1) ฤ (ร.รึ)[rue] is used with a words that came from Bali and Sansakrit by the rule of reading as follows.
A. Pronunciation is "RI" when combined with consonants (ก ต ท ป ศ ส), for instant, กฤษฎีกา[kritsadika] กฤษณา[kritsana] ตฤณมัย[kinnamai] ทฤษฎี[tritsadi] ปฤษฎางค์ [prissadang] ปฤจฉา[prijsha] ศฤงคาร[saringkaan] ฤทธิ[rit] วิกฤติกาล[wikritkaan] รังสฤษภ์ [rangsarit] อังกฤษ[angkrit] อมฤต[amarit] and ธนกฤต[thanakrit]--which is my name.
B. Pronunciation is "RUE" while mixing of consonants (ด น พ ม ห), for example, คฤหัสถ์[karuehat] คฤหาสน์[karuehaat] นฤมล[naruemon] พฤษภาคม[pruessapa] พฤกษชาติ[prueksachaat] หฤทัย[haruetahai] หฤหรรษ์[haruehan]
C. Pronunciation is "RER" (short sound), such as ฤกษ์[rerk] ฤกเษก[rerksek].
D. The rule of using of ฤา (รือ)[RUEA] (long soung), in Thai words are used very little, like, ฤาทัย[rueatahi] ฤาษี[rueasi]. Currently, it can only find in poetry, such as poem, verse and traditional thai singing drama. Mostly, today, it is used in form of ฤา (Negative form), for instant, ฤาใคร่วายวาง[rueakraiwaiwang] ฤาพระจักละน้อง[ruea-pra-chak-la-nong], สองเขือพี่หลับไหล[song-khuea-pi-laab-lai] ลืมตื่น[lueam-tuean] ฤาพี่[ruea-pi].

2.2) ฦ (ล.ลึ)[lue]
a. The using of ฦ (ลึ)[LUE] (short sound) which have just found in ancient Thai language, such as, น้ำฦก[namluek] ฦ[luek] that nowaday is obsolette.
b. The last word is ฦา (ลือ)[LUE] (long soung) having appeared in ancient Thai words only, for example, เลื่องฦา[lueng-lue] เล่าฦา[lao-lue]. It is as same as the above example that is out of date.