The Glottal Stop

The glottal stop is a plosive created by complete closure and then opening of the glottis (vocal folds). The symbol for this sound is: [ ʔ ] , a sort of question mark without a dot at the bottom.

Although it is a consonant phoneme in many languages, e.g. Hebrew and Arabic, in English the glottal stop generally appears as an allophone of /t/. This is called Glottal Replacement and is most noticeable in the form that it takes in several regional accents of British English (e.g. Cockney, Glasgow), where syllable-final /t/ between two vowels is replaced by [ʔ]. For example:

Better [ˈbeʔə]
Fitting [ˈfɪʔɪŋ]
A bit of butter [ə ˈbɪʔ ə ˈbʌʔə]

N.b. these pronunciations are not recommended to learners of English

While the above examples are generally not considered acceptable in Standard British English, in other contexts the glottal stop is ever more frequently heard.

Such contexts are: syllable final /t/ following a vowel or a voiced consonant and the following sound is:

A stop or a fricative, e.g.
Football /ˈfʊtbɔ:l/ => [ˈfʊʔbɔ:l]
Hit them /ˈhɪt ðəm/ => [ˈhɪʔ ðəm]
Anthill /ˈænthɪl/ => [ˈænʔhɪl]
A nasal e.g.
Fitness /ˈfɪtnəs/ => [ˈfɪʔnəs]
Utmost /ˈʌtməʊst/ => [ˈʌʔməʊst]
White mice /waɪt ˈmaɪs/ => [waɪʔ ˈmaɪs]
A semi-vowel or non-syllabic /l/
Atlas /ˈætləs/ => [ˈæʔləs]
Dauntless /ˈdɔ:ntləs/ => [ˈdɔ:nʔləs]
Quite well /kwaɪt ˈwel/ => [kwaɪʔ ˈwel]

Another phenomenon concerning the glottal stop is that of Glottal Reinforcement. This concerns the sound /tʃ/ at the end of a syllable, e.g.

Teacher /ˈti:tʃə/ => [ˈti:ʔtʃə]
Fetching /ˈfetʃɪŋ/ => [ˈfeʔtʃɪŋ]
Reaches

/ˈri:tʃɪz/ => [ˈri:ʔtʃɪz]

A similar process may involve /p/, /t/ or /k/ if followed by a consonant, or in work final position.

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For more information see:

a document in John Wells' pages

and the pages on Wikipedia on the Glottal Stop and Glottalization.