My wife tells me that her teachers taught her (some 50 years ago) another tone mark besides the 4 standard ones ไม้เอก ไม้โท ไม้จัตวา ไม้ตรี. Unlike the common four marks, the fifth mark wasn't over a word, but over the hand or over the butt. Known as the ไม้เรียว. She says that was plenty incentive to learn the proper spelling/pronunciation of words in her early primary school days. If I'm not mistaken, the ไม้เรียว is no longer used in Thai schools.

Okay, back to more serious Thai studying.

ไม้เรียว is actually still widely employed as part of the curriculum at most schools, especially those upcountry. The sound is best described as a "whooshing", followed by a short, sharp "crack" at the end, and it serves to intensify the meaning of words so that they may not be easily forgotten.