U+1A78 Tai Tham Sign Khuen Tone-4

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Tai Tham script (Tham meaning "scripture") is an abugida writing system used mainly for a group of Southwestern Tai languages i.e., Northern Thai, Tai Lü, Khün and Lao; as well as the liturgical languages of Buddhism i.e., Pali and Sanskrit. It is historically known as Tua Tham (ᨲ᩠ᩅᩫᨵᨾ᩠ᨾ᩼ or ᨲ᩠ᩅᩫᨵᩢᨾ᩠ᨾ᩼). In Thailand and Myanmar, the script is often referred to as Lanna script (Thai: อักษรธรรมล้านนา RTGS: Akson Tham Lan Na; Burmese: လန်နာအက္ခရာ; MLCTS: Lanna Akkhara) in relation to the historical kingdom of Lan Na situating in the Northern region of modern day Thailand and a part of Shan state in Myanmar. Local people in Northern Thailand also call the script as Tua Mueang (ᨲ᩠ᩅᩫᨾᩮᩥᩬᨦ, Northern Thai pronunciation: [tǔa̯.mɯ̄a̯ŋ] ) in parallel to Kam Mueang, a local name for Northern Thai language. In Laos and Isan region of Thailand, a variation of Tai Tham script, often dubbed Lao Tham, is also known by the locals as To Tham Lao (Northeastern Thai: โตธรรมลาว /toː˩.tʰam˧˥.laːw˧/, cf. Lao: ໂຕທຳ/ໂຕທັມ BGN/PCGN to tham) or Yuan script. Tai Tham script is traditionally written on a dried palm leaf as a palm-leaf manuscript.

The Northern Thai language is a close relative of (standard) Thai. It is spoken by nearly 6 million people in Northern Thailand and several thousand in Laos of whom few are literate in Lanna script. The script is still read by older monks. Northern Thai has six linguistic tones and Thai only five, making transcription into the Thai alphabet problematic. There is some resurgent interest in the script among younger people, but an added complication is that the modern spoken form, called Kam Muang, differs in pronunciation from the older form.

There are 670,000 speakers of Tai Lü, some of those born before 1950 are literate in Tham, also known as Old Tai Lue. The script has also continued to be taught in the monasteries. The New Tai Lue script is derived from Tham. There are 120,000 speakers of Khün for which Lanna is the only script.