Vocal Cord Action

Three main actions are distinguished: vibrating, for voiced sounds, e.g. ah, zzzzzzz; not vibrating, for voiceless sounds, e.g. h, or ssssss; and stopped, in the glottal stop, as in wa'er, bu'er....

Glottal stop occurs in wa'er where most educated speakers would look askance at it, but it quite often occurs much more respectably and less noticeably as, for example, a substitute for the t in not much. In both cases closure of the vocal cords and their explosive opening constitute the primary articulation of the sound; this is true too of h where the vocal cords come close enough together to produce friction. The voiced/voiceless distinction, however, is almost always an accompaniment to some other articulation in the pharynx or mouth. In s voicelessness accompanies the main articulation by the blade of the tongue, and in z the same blade articulation is accompanied by voice.

Glottal stop often occurs, even more respectably, at the beginning of a word like Idiot! said vehemently. Here it serves simply as a hard beginning to the vowel sound for purposes of emphasis.

(from O'Connor, Phonology, 1973).