How did linguists use vocabulary similarities to identify the three Tai language groups?

Linguists classify the Tai languages into three distinct groups—Southwestern, Central, and Northern—by analyzing lexical similarities and phonological patterns.1

The identification of these groups relies on the distribution of shared vocabulary items. While a large number of words are shared across all three groups, suggesting a common genetic relationship, linguists also identify specific patterns where vocabulary is shared by only two of the three groups. For example:21

• Shared by Southwestern and Central groups: The word for "sky" (e.g., Thai fáa and Longzhou faa) is used in these groups but replaced by a different term in the Northern group.2
• Shared by Central and Northern groups: The word for "beard" (e.g., Longzhou mum and Buyei mum) is shared by these groups but replaced in the Southwestern group.2
• Shared by Southwestern and Northern groups: The term for "knife" (e.g., Thai mîit and Buyei mit) is shared by these groups but not by the Central dialects.2

These specific lexical overlaps, combined with phonological evidence, provide the basis for distinguishing the three groups.21