Later a 45th symbol was added to accommodate accent variation, a form of diaphonemic writing. In the original set, a "hook a" or "two-storey a" (a) was used for the vowel in "cat" (lexical set TRAP), and a "round a" or "one-storey a" (ɑ) for the sound in "father" (lexical set PALM). But lexical set BATH (words such as "rather", "dance", and "half") patterns with PALM in some accents including Received Pronunciation, but with TRAP in others including General American. So a new character, the "half-hook a", was devised, to avoid the necessity of producing separate instructional materials for speakers of different accents.