The term stiff voice describes the pronunciation of consonants or vowels with a glottal opening narrower, and the vocal folds stiffer, than occurs in modal voice.
[1][page needed] In Bru, for example, stiff-voiced vowels have tenseness in the glottis and pharynx without going so far as to be creaky voiced, whereas slack-voiced vowels are lax in the glottis without going so far as to be breathy voice.
[2] One language with stiff voice is Thai:[1] Javanese contrasts stiff and slack voiced bilabial, dental, retroflex, and velar stops.
[1] Mpi (Loloish) contrasts modals and stiff voice in its vowels.
This is not register: for each of the six Mpi tones, a word may have either a modal or stiff-voiced vowel.