Received Pronunciation (RP) holds prestige not only in the UK, but also in many Outer and Expanding Circle countries and, even in the US, but the TRAP-BATH split (TBS) in RP, which refers to the shift from the vowel /a/ to /ɑː/, has still remained ambiguous because this sound shift contains inconsistent realizations that occur in identical codas, as illustrated in such lexical pairs as class but gas, path but math, last but enthusiast . What makes this sound feature more complicated is sociolinguistic variation. Although earlier discoveries have enhanced our understanding of the TBS, neither a definitive word list nor lexical frequency are provided for learners to enunciate BATH words. To provide RP learners with a linguistic index to learn BATH lexical set, this study expanded Wells’s study by including additional 16 phonetic environments, which generated a total of 304 relevant words for examining their pronunciation defined by dictionaries. The findings show that 13 out of the 30 environments can trigger the TBS more than 50% of the time. In particular, seven of them can completely predict the TBS (over 91%), and one of them can fully do so (83%). This feature appears as a regular conditioned sound pattern if well-defined by its phonetic environments.