[...] linguists usually distinguish between two varieties, known as clear and dark [...]. The two varieties never contrast with each other in English and so are never responsible for creating differences in meaning. This is because the two varieties are distributed differently in English words; (60)Certain varieties of English such as General American, Canadian, Glaswegian English (as well as some varieties of Australian English) predominantly have dark /l/ in almost all contexts. Cum grano salis the last sentence in the above quotation is acceptable if we restrict its applicability to what PT calls SESP (aka RP, GB, ...).
PT introduces the terms allophone and phoneme, the distinction between broad and narrow transcription and the one between slant and square brackets. Coming back to the two /l/-sounds, he describes their distribution (in SESP!):
[...] the clear [l] occurs before vowels and the consonant /j/, whereas the dark [ɫ]/[lˠ] occurs before consonants, except /j/, and at the end of words. (61)This sentence is a bit vague as it stands, I think. Clear l occurs before vowels even if the /l/-phoneme ends a word but abuts with an initial vowel of a following word and both words are slurred. As regards the distribution of the dark variant, one must not forget to mention syllabic l, which is dark as well. One might even say that in many (all?) varieties of English there is not a binary distinction but rather a continuum from light to dark l (see John Maidment's blog of the 13th of May, 2011).
PT then illustrates various other allophone variants of phonemes such as
- aspirated and unaspirated plosives, e.g. [ph, p˺]
- glottal reinforcement and glottal replacement of plosives, e.g. [ʔp, ʔ]
- flapping of /t/, e.g. but I as [bʌt̬ aɪ]
- devoicing of canonically voiced consonants, e.ɡ. [b̥]
- fronting, backing and rounding of consonants in their respective environments
- nasalisation of vowels adjacent to nasal consonants
- pre-fortis clipping of vowels
- pre-l breaking and pre-r breaking as in [fiːəɫ] and [ˈsɪəriəs]
- smoothing of a long monophthong or diphthong plus a weak vowel as when liable becomes [ˈlaəbɫ]
- diphthongisation of /i:, u:/, e.ɡ. in [thɪi] or [thʊu]
I shall still recommend this book to my students, but with some accompanying comments.
Addendum: See also Alex Rotatori's critical evaluation here.