It is never universal, especially in careful speech[citation needed], and it most often alternates with other allophones of /t/ such as [t]ⓘ, [tʰ], [tⁿ] (before a nasal), [tˡ] (before a lateral), or [ɾ].
[2] The SED fieldworker Peter Wright found it in areas of Lancashire and said, "It is considered a lazy habit, but may have been in some dialects for hundreds of years.
[5] David Crystal claims that the sound can be heard in Received Pronunciation (RP) speakers from the early 20th century such as Daniel Jones, Bertrand Russell and Ellen Terry.
[6] The Cambridge English Pronouncing Dictionary claims that t-glottalization is now most common in London, Leeds, Edinburgh, and Glasgow.
[12][13] In RP, and in many accents such as Cockney, it is common for /t/ to be completely replaced by a glottal stop before another consonant,[14][15] as in not now [nɒʔnaʊ] and department [dɪpɑː(ɹ)ʔmənʔ].
[19] One study carried out by Anne Fabricius suggests that t-glottalization is increasing in RP, the reason for this being the dialect levelling of the Southeast.
[22] T-glottalization, especially at word boundaries, is considered both a geographic and sociolinguistic phenomenon, with rates increasing both in the western U.S. and in younger female speakers.