The trap-bath split refers to a sound change which occurred in the English of southern England. It is believed that this sound change took place in the late 1700's. The word "trap" has the low front unrounded vowel and "bath" has the low back unrounded vowel. In my Canadian English, both "trap" and bath" are pronounced with a front vowel.
Actually, the low front unrounded [ae] vowel in words such as "cat" and "that" initially lengthened and then became a back vowel. In northern England, this sound change never occurred. It may be argued that the low back vowel is less marked than the low front vowel. However, in languages such as Spanish and Italian, the "a" is not a front or back vowel but rather a central vowel.
In which environment did this sound change occur? It occurred before usually tautosyllabic voiceless (the syllable-final vowel of "castle" is an exception) consonants and nasals. "Rather", if it is an example of this sound change, is also an exception because the "th" is in the following syllable and is voiced. However, this rule does not apply in all situations. For example, the front [ae] occurs in "lass", "mass", "gas", "fan", "hand" and "maths". In some cases, the use of the back vowel is optional as in "trans" in "trans-Atlantic" and in "circumstance". Some speakers use the front vowel and others the back.
It does not occur before the velar "k" or the alveopalatals "sh" and "ch". Thus, it appears that this sound change only applied before +anterior consonants.
I made a list of 75 words in which the back vowel occurs in speakers of English dialects which have the trap-bath split. This is true of Received Pronunciation and other dialects of England as well as Australian, New Zealand and South African English. In the USA, the trap-bath split can be heard in the Boston accent, but for the overwhelming majority of American speakers, there is no trap-bath split.
Here is my list of 75 words:
sample, path, aunt, calf, rather
shaft, raft, master, plaster, plant
advantage, advance, answer, giraffe, daft
graft, Alexander, demand, Sandra, slander
chancellor, enhance, trance, stance, ranch
chant, enchant, slant, brass, fasten
ask, branch, cast, past, last
fast, mast, path, pass, glass
grass, task, mask, dance, chance
can't, rather, bath, laugh, France
half, class, blast, after, craft
draft, graph, staff, castle, aunt
fasten, basket, casket, flask, rascal
blast, contrast, nasty, pastor, vast
disaster, grasp, grant, command, transport
I then determined the frequency of occurrence of the back vowel before each consonant. Here are my results (percentages are not exact due to rounding) :
before s: 31 occurrences [42%]
before f: 12 occurrences [16%]
before n: 28 occurrences [37%]
before th: 2 occurrences [3%] (this is the voiceless fricative)
before m: 1 occurrence [1%]
before th: 1 occurrence [1%] (this is the voiced fricative)
What do these results suggest? They suggest that the sound change in which the low front vowel became a back front one occurred most often before an "s" (42%) or an "n" (37%). Together these two consonants account for 79% of the examples in the data.
The "s" and "n" are similar in the sense that they are both alveolars- they share the same place of articulation. Also, they are both produced with the tongue tip, making them coronals, and they are both +anterior sounds. The "n" is not voiceless but we do not usually need to specify voicing in nasals because they are nearly always voiced.
The trap-bath split is a very interesting phenomenon which occurred in the history of the English language. Surprisingly, though, I never studied this when I took linguistics at university. I learned this on my own.