How are Tai languages divided into Southwestern, Central, and Northern groups?

The classification of Tai languages into Southwestern, Central, and Northern groups is based on linguistic relationships proposed in 1959–60, utilizing lexical (vocabulary) and phonological (sound system) criteria.1

• Southwestern Group: This is the most geographically widespread division, encompassing about two-thirds of the Tai-speaking population. It includes languages such as Thai, Lao, Shan, Khün, Lü, White Tai, and Black Tai, spoken across Thailand, Laos, Myanmar, northern Vietnam, and Yunnan, China.12
• Central Group: This group consists of the Tay dialects of northern Vietnam and various dialects in Guangxi, China, such as Longzhou.1
• Northern Group: This group includes the Buyei dialects of Guizhou and the Zhuang dialects of Guangxi, China. Some Northern dialects are also found in Yunnan and Vietnam, while the Saek language is spoken as far south as Laos and Thailand.1

These groups are further distinguished by shared vocabulary patterns and distinct writing systems: the Central and Northern dialects often use scripts derived from Chinese, while many Southwestern languages utilize scripts derived from Indic sources.34