Abstract:
To analyze and compare the split and merger patterns of tones B and C in the P and PH language groups of Southwestern Tai (SWT). Based on the development of tones *B and *C, 15 languages were subclassified. Finally, Bangkok Thai words (Siamese) containing the falling tone were compared with those of Proto-Southwestern Tai (PSWT) having tones *B and *C in order to check the spelling of the Bangkok Thai words. The data used in this study were elicited from the available wordlists and dictionaries of 15 Southwestern Tai languages and also from the researcher's field notes. These 15 languages were divided into two groups: 8 languages of the P group (Black Tai, White Tai, Tai Muang Toek, Red Tai, Tai Lue, Tai Yuan, Tai Phakae and Tai Nue) and 7 languages of the PH group (Bangkok Thai, Lao, Southern Thai, Tak Bai, Phuthai, Puan and Yo). The findings reveal that there is only one prominent pattern of the development of *B and *C in the P group, *B > B123-4 and *C > C123-4. The patterns found in the PH group are more complicated. For each tone, three patterns have been found. The prominent patterns are *B > B123-4 and *C > C123-4. Regarding the patterns of development of *B together with *C, five patterns have been found. The combinative pattern *B > B123-4 *C > C123-4 and B4 not equal C123 is the most prominent. Besides, Lao, Yo and Southern Thai have distinctive patterns, i.e., *B > B1234 *C > C1-234 and B4 not equal C123 in Lao, *B > B123-4 *C > C1-234 B234 = C4 and B123 = C1 in Yo, and *B > B1-23-4 *C > C1-23-4 and B4 = C123 in Southern Thai, Bangkok Thai, a language in the PH group and Red Tai, a language in the P group, have the same type of combinative development pattern, *B > B123-4 *C > C123-4 and B4 =C123. The findings suggest that the development of tone *B and of tone *C should not be used as the criteria for subcategorizing languages in the P group, even though they seem to work quite well for languages in the PH group. The PH-group languages can be subclassified into 3 subgroups: a) Bangkok Thai, Phuthai, Puan and Takbai, b) Lao and Yo, and c) Southern Thai. When the Bangkok Thai words containing the falling tone, were compared with the reconstructed PSWT forms having tone *B and *C, 28 of them have been found misspelled since their tones do not conform to the development of tones *B and *C