How did Central Tai dialects change some Proto-Tai consonant clusters?

In the study of Tai languages, the Central Tai dialects are distinguished by their specific treatment of certain Proto-Tai consonant clusters, such as tr*- and thr*-.

While other Tai dialect groups typically maintain a distinction between these protoforms—for example, keeping them separate as seen in Thai or Buyei—the Central Tai dialects have undergone a phonological change that causes these clusters to merge into a single sound. This merger is evidenced by comparing forms across different groups; for instance, while other dialects may preserve distinct sounds, Central dialects like Tay and Longzhou reflect the merger by using identical initial sounds for words that were originally distinct in the proto-language.1