Glottalization is the complete or partial closure of the glottis during the articulation of another sound. Glottalization of vowels and other sonorants is most often realized as creaky voice (partial closure). Glottalization of obstruent consonants usually involves complete closure of the glottis; another way to describe this phenomenon is to say that a glottal stop is made simultaneously with another consonant. In certain cases, the glottal stop can even wholly replace the voiceless consonant. The term 'glottalized' is also used for ejective and implosive consonants; see glottalic consonant for examples.
There are two other ways to represent glottalization of sonorants in the IPA: (a) the same way as ejectives, with an apostrophe; or (b) with the under-tilde for creaky voice. For example, the Yapese word for "sick" with a glottalized m could be transcribed as either [mʼaar] or [m̰aar]. (In some typefaces, the apostrophe will occur above the m.)
Glottalization varies along three parameters, all of which are continuums. The degree of glottalization varies from none (modal voice, [d]) through stiff voice ([d̬]) and creaky voice ([d̰]) to full glottal closure (glottal reinforcement or glottal replacement, described below). The timing also varies, from a simultaneous single segment [d̰] to an onset or coda such as [ˀd] or [dˀ] to a sequence such as [ʔd] or [dʔ]. Full or partial closure of the glottis also allows glottalic airstream mechanisms to operate, producing ejective or implosive consonants; implosives may themselves have modal, stiff, or creaky voice. It is not always clear from linguistic descriptions if a language has a series of light ejectives or voiceless consonants with glottal reinforcement, or similarly if it has a series of light implosives or voiced consonants with glottal reinforcement. The airstream parameter is only known to be relevant to obstruents, but the first two are involved with both obstruents and sonorants, including vowels.
When a phoneme is completely substituted by a glottal stop [ʔ], one speaks of glottaling or glottal replacement. This is, for instance, very common in Cockney and Estuary English. In these dialects, the glottal stop is an allophone of /p/, /t/ and /k/ word-finally, and when followed by an unstressed vowel (including syllabic /l/ /m/ and /n/) in a post-stress syllable. 'Water' can be pronounced [ˈwɔːʔə] – the glottal stop has superseded the 't' sound. Other examples include "city" [ˈsɪʔi], "bottle" [ˈbɒʔʊw], "Britain" [ˈbɹɪʔən], "seniority" [sɪiniˈɒɹəʔi]. In some consonant clusters, glottal replacement of /t/ is common even among RP speakers.
Glottal replacement also occurs in Indonesian, where syllable final /k/ is produced as a glottal stop. In Hawaiian, the glottal stop is reconstructed to have come from other Proto-Polynesian consonants. The following table displays the shift /k/ → /ʔ/ as well as the shift /t/ → /k/.
Glottal replacement is not purely a feature of consonants. Yanesha' has three vowel qualities (/a/, /e/, and /o/) that have phonemic contrasts between short, long, and "laryngeal" or glottalized forms. While the latter generally consists of creaky phonation, there is some allophony involved. In pre-final contexts, a variation occurs (especially before voiced consonants) ranging from creaky phonation throughout the vowel to a sequence of a vowel, glottal stop, and a slightly rearticulated vowel: /maˀˈnʲoʐ/ ('deer') → [maʔa̯ˈnʲoʂ].
When a phoneme is accompanied (either sequentially or simultaneously) by a [ʔ], then one speaks of pre-glottalization or glottal reinforcement.
This is common in some varieties of English, RP included; /t/ and /tʃ/ are the most affected but /p/ /k/ also regularly show pre-glottalization. In the English dialects exhibiting pre-glottalization, the consonants in question are usually glottalized in the coda position: "what" [ˈwɒʔt], "fiction" [ˈfɪʔkʃən], "milkman" [ˈmɪlʔkmən], "opera" [ˈɒʔpɹə]. To a certain extent, some types of English have free variation between glottal replacement and glottal reinforcement.
Glottal reinforcement may occasionally be observed in more conservative varieties of Tweants, a dialect of the Dutch Low Saxon language group. It almost exclusively concerns the /t/ consonant, and is sometimes orthographically represented by a double -t. It mostly occurs in monosyllabic verbs, such as "iej loatt" (you let) en "wie weett" (we know). Sometimes it may also occur in contracted words, such as in the phrase: "Wo geet t?" (lit. how goes it?), which is pronounced [ʋɔ ˈɣeːʔtˢ].