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In phonetics, voice onset time, commonly abbreviated VOT, is the length of time that passes between when a consonant is released and when voicing, the vibration of the vocal folds begins.

The three major phonation types of stops can be analyzed in terms of their voice onset time.

• Simple unaspirated voiceless plosives, sometimes called tenuis plosives, have a voice onset time at or near zero, meaning that the voicing of a following sonorant (such as a vowel) begins at or near to when the stop is released. (An offset of 15 ms on [t] or 30 ms on [k] is inaudible, and counts as tenuis.)

• Aspirated plosives followed by a sonorant have a voice onset time greater than this amount, called a positive VOT. The length of the VOT in such cases is a practical measure of aspiration: The longer the VOT, the stronger the aspiration. In Navajo, for example, which is strongly aspirated, the aspiration (and therefore the VOT) lasts twice as long as it does in English: 160ms vs. 80ms for [kʰ], and 45ms for [k]. Some languages have weaker aspiration than English. For velar stops, tenuis [k] typically has a VOT of 20-30 ms, weakly aspirated [k] of some 50-60 ms, moderately aspirated [kʰ] averages 80-90 ms, and anything much over 100 ms would be considered strong aspiration. (Another phonation, breathy voice, is commonly called voiced aspiration; in order for the VOT measure to apply to it, VOT needs to be understood as the onset of modal voicing. Of course, an aspirated consonant will not always be followed by a voiced sound, in which case VOT cannot be used to measure it.)

• Voiced plosives have a voice onset time noticeably less than zero, a negative VOT, meaning the vocal cords start vibrating before the stop is released. With a fully voiced stop, the VOT coincides with the onset of the stop; with a partially voiced stop, such as English [b, d, g] in initial position, voicing begins sometime during the closure (occlusion) of the consonant.

Because neither aspiration nor voicing is absolute, with intermediate degrees of both, the relative terms fortis and lenis are often used to describe a binary opposition between a series of consonants with higher (more positive) VOT, defined as fortis, and a second series with lower (more negative) VOT, defined as lenis. Of course, being relative, what fortis and lenis mean in one language will not in general correspond to what they mean in another.

Voicing contrast applies to all types of consonants, but aspiration is generally only a feature of stops and affricates.

• Taehong Cho and Peter Ladefoged, "Variations and universals in VOT". In Fieldwork Studies of Targeted Languages V: UCLA Working Papers in Phonetics vol. 95. 1997.