Trap–bath split

The TRAP–BATH split is a vowel split that occurs mainly in Southern England English (including Received Pronunciation), Australian English, New Zealand English, Indian English and South African English; it also occurs to a lesser extent in some Irish English and Welsh English as well as older Northeastern New England English. The split entails the Early Modern English phoneme /æ/ lengthening in certain environments and ultimately merging with the long /ɑː/ of PALM. In that context, the lengthened vowel in words such as bath, laugh, grass and chance in accents affected by the split is referred to as a broad A (also called in Britain long A). Phonetically, the vowel is [ɑː] in Received Pronunciation (RP), Cockney and Estuary English; in some other accents, including Australian and New Zealand accents, it is a more fronted vowel ([ɐː] or [aː] ) and tends to be a rounded and shortened [ɒ~ɔ] in Broad South African English. A trap–bath split also occurs in the accents of the Middle Atlantic United States (New York City, Baltimore, and Philadelphia accents), but it results in very different vowel qualities from the aforementioned British-type split. To avoid confusion, the Middle Atlantic American split is usually referred to in American linguistics as a 'short-a split'.

In accents unaffected by the split, words like bath and laugh usually have the same vowel as words like cat, trap and man: the short A or flat A. Similar changes took place in words with ⟨o⟩ in the lot–cloth split.

The sound change originally occurred in Southern England and ultimately changed the sound of /æ/ to /ɑː/ in some words in which the former sound appeared before /f, s, θ, ns, nt, ntʃ, mpəl/. That led to RP /pɑːθ/ for path, /tʃɑːnt/ for chant etc. The sound change did not occur before other consonants and so accents affected by the split preserve /æ/ in words like cat. (See the section below for more details on the words affected.) The lengthening of the bath vowel began in the 17th century but was "stigmatised as a Cockneyism until well into the 19th century".