Six-hour clock

The six-hour clock is a traditional timekeeping system used in the Thai and formerly the Lao language and the Khmer language, alongside the official 24-hour clock. Like other common systems, it counts twenty-four hours in a day, but divides the day into four quarters, counting six hours in each. The hours in each quarter (with the exception of the sixth hour in each quarter) are told with period-designating words or phrases, which are:

These terms are thought to have originated from the sounds of traditional timekeeping devices. The gong was used to announce the hours in daytime, and the drum at night. Hence the terms mong, an onomatopoeia of the sound of the gong, and thum, that of the sound of the drum. Ti is a verb meaning to hit or strike, and is presumed to have originated from the act of striking the timekeeping device itself.Chao and bai translate as morning and afternoon respectively, and help to differentiate the two daytime quarters.

The sixth hours of each quarter are told by a different set of terms. The sixth hour at dawn is called yam rung (ย่ำรุ่ง, [jâm rûŋ]), and the sixth hour at dusk is called yam kham (ย่ำค่ำ, [jâm kʰâm]), both references to the act of striking the gong or drum in succession to announce the turning of day (yam), where rung and kham, meaning dawn and dusk, denote the time of these occurrences. The midday and midnight hours are respectively known as thiang (เที่ยง, [tʰîaːŋ], or thiang wan, เที่ยงวัน, [tʰîaːŋ wān]) and thiang khuen (เที่ยงคืน, [tʰîaːŋ kʰɯ̄ːn]), both of which literally translate as midday and midnight.

Midnight is also called song yam (สองยาม, [sɔ̌ːŋ jāːm]; note that yam is a different word), a reference to the end of the second three-hour period of the night watch (song translates as the number two). In addition, hok (6) thum and ti hok may also be used to refer to the hours of midnight and dawn, following general usage for the other hours, although more rarely; and the fourth to sixth hours of the second daytime half may also be told as ...mong yen (...โมงเย็น, [mōːŋ jēn]), yen meaning evening.

...
Wikipedia