ศาลาชุมนุม

I can't believe I've never noticed this before. What's the purpose of upper case B on the Thai keyboard?
It puts a circle below a previously typed consonant.
My wife seems to think it has some link to Sanskrit?

This element of Thai orthography is called a "อฺ Phinthu (พินทุ )" or "พินธุจุด"

This archaic orthography is nowadays mostly used in dictionaries to disambiguate pronunciation. In our online dictionary, the symbol, if used, only appears in a word at the top of its main definition page. The symbol can be used in two ways:

Placed underneath the first consonant of an orthographic consonant cluster which may in turn be one of three cases:

a. A double-consonant cluster
b. Silent ห, called หอ นำ /haawR namM/
c. Silent อ, called ออ นำ /aawM namM/

Placed underneath the final-sounding consonant of a syllable.

Both of these rules can be summarized "more snappily," as Richard Wordingham says, by noting that "Phinthu indicates that there is no vowel sound associated with the [marked] consonant." These seemingly vague rules can be puzzling; for clarity, our online dictionary will only place พินทุ /phinM thooH/ under ห or อ when that consonant is acting in its silent role.

Examples of พินทุ :

ขฺบวน /khaL buaanM/
เพลฺ /phaehnM/
แหนฺ /haaenR/
แหฺม /maaeR/
โหนฺ /ho:hnR/
อินฺทฺนนท์ /inM thaH nohnM/

This explanation is from this website at http://www.thai-language.com/ref/special-symbols. Scroll down the page until you come to the relevant section.

Thank you for the reply.
I'm struggling to understand the way it's explained. Perhaps I've missed something?
One of the examples: แหนฺ haaenR and yet the English has the 'n' sound at the end of the word for which I thought the นฺ was now silent?

bahtman wrote:Thank you for the reply.
I'm struggling to understand the way it's explained. Perhaps I've missed something?
One of the examples: แหนฺ haaenR and yet the English has the 'n' sound at the end of the word for which I thought the นฺ was now silent?

The spelling of the Thai word "แหน" is on its face ambiguous. It could be pronounced "้haen [rising tone]" or "naeh [rising tone]". In other words the leading consonant "ห" could serve either as a sounded consonant or as a tone indicator.

The Royal Thai Institute Dictionary uses the traditional pintu to indicate pronunciation. Since the RID does not use international phonetic notations, the use of the pintu is the only way it has to indicate pronunciation in these ambiguous cases. Unfortunately the use of the pintu seems to have fallen into disuse almost everywhere else.

These are entries from the RID:
________________________________________________________
"แหน ๑
[แหนฺ] ก. ใช้เข้าคู่กับคำอื่น ในคำว่า หวงแหน แห่แหน เฝ้าแหน."

"แหน ๔
[แหฺน] น. ชื่อไม้นํ้าหลายชนิดในวงศ์ Lemnaceae ใบกลมเล็ก ๆ ลอยอยู่ตามนํ้านิ่ง เช่น แหนเล็ก (Lemna aequinoctialisWelw.) แหนใหญ่ [Spirodela polyrrhiza (L.) Schleid.], แหนแดง ก็เรียก.
_________________________________________________________

I still don't get it David. How about this example:
เพลฺ /phaehnM/
I would read that without the phinthu as 'pain', with the phinthu as 'pay', as the ล is now silent?

bahtman wrote:I still don't get it David. How about this example:
เพลฺ /phaehnM/
I would read that without the phinthu as 'pain', with the phinthu as 'pay', as the ล is now silent?

The alternative pronunciation would be "เพฺล" [phlae]. Compare this from the RID:

"แผล
(๑) [แผฺล] น. เนื้อหนังที่แตกแยกออกเพราะเป็นโรคหรือถูกของมีคมเป็นต้น เช่น ขาเป็นแผล"

In this pronunciation convention with the pintu under the "ผ", a cluster is indicated and the pronunciation is [phlaeh [rising tone]]

Here are some more examples from pages 511-512 of "หลักภาษาไทย" by กำชัย ทองหล่อ, พ.ศ. 2545 [newer editions may have different page numbers]:

คำ "เพลา" - อ่าน "เพฺลา" (ควบ) ๑. แกนสำหรับสอดในดุมให้ล้อหมุน" ... [หรือ] อ่าน "เพ-ลา" [หมายถึง] กาล, คราว, เวลา" [phlau vs. phae-lah]

คำ "โคลง" - อ่าน "โคฺลง" (ควบ) ... [หรือ] อ่าน "โค-ลง" [khlohng vs. khloh-long]

คำ "กรี" - อ่าน "กะรี" ... [หรือ] อ่าน"กฺรี" (ควบ) [ka-ree vs. kree]

คำ "เสลา" - อ่าน "เสฺลา" (ส-นำ) ... [หรือ] อ่าน"เส-ลา" [salao [rising tone] vs sae [rising] - lah [mid]]

And, that source has even more examples.