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Lip reading, also known as lipreading or speechreading, is a technique of understanding speech by visually interpreting the movements of the lips, face and tongue when normal sound is not available, relying also on information provided by the context, knowledge of the language, and any residual hearing. Although primarily used by deaf and hard-of-hearing people, it may under some circumstances be used by people with normal hearing.
In everyday conversation, people with normal vision, hearing and social skills sub-consciously use information from the lips and face to aid aural comprehension and most fluent speakers of a language are able to speechread to some extent (see McGurk effect). This is because each speech sound (phoneme) has a particular facial and mouth position (viseme), and people can to some extent deduce what phoneme has been produced based on visual cues, even if the sound is unavailable or degraded (e.g. by background noise).
Speechreading is limited, however, in that many phonemes share the same viseme and thus are impossible to distinguish from visual information alone. Sounds whose place of articulation is deep inside the mouth or throat are not detectable, such as glottal consonants and most gestures of the tongue. Voiced and unvoiced pairs look identical, such as [p] and [b], [k] and [g], [t] and [d], [f] and [v], and [s] and [z] (in American English); likewise for nasalisation (e.g. [m] vs. [b]). It has been estimated that only 30% to 40% of sounds in the English language are distinguishable from sight alone.
Thus, for example, the phrase "where there's life, there's hope" looks identical to "where's the lavender soap" in most English dialects. Author Henry Kisor titled his book What's That Pig Outdoors?: A Memoir of Deafness in reference to mishearing the question, "What's that big loud noise?" He used this example in the book to discuss the shortcomings of speechreading.
As a result, a speechreader must depend heavily on cues from the environment, from the context of the communication, and a knowledge of what is likely to be said. It is much easier to speechread customary phrases such as greetings or a connected discourse on a familiar topic than utterances that appear in isolation and without supporting information, such as the name of a person never met before.
Difficult scenarios in which to speechread include:
- Lack of a clear view of the speaker's lips. This includes:
- obstructions such as moustaches or hands in front of the mouth
- the speaker's head turned aside or away
- dark environment
- a bright back-lighting source such as a window behind the speaker, darkening the face.
- Group discussions, especially when multiple people are talking in quick succession. The challenge here is to know where to look.
Use of speechreading by deaf people 
Speechreaders who have grown up deaf may never have heard the spoken language and are unlikely to be fluent users of it, which makes speechreading much more difficult. They must also learn the individual visemes by conscious training in an educational setting. In addition, speechreading takes a lot of focus, and can be extremely tiring. For these and other reasons, many deaf people prefer to use other means of communication with non-signers, such as mime and gesture, writing, and sign language interpreters.
To quote from Dorothy Clegg's 1953 book The Listening Eye, "When you are deaf you live inside a well-corked glass bottle. You see the entrancing outside world, but it does not reach you. After learning to lip read, you are still inside the bottle, but the cork has come out and the outside world slowly but surely comes in to you." This view—that speechreading, though difficult, can be successful—is relatively controversial within the deaf world; for an incomplete history of this debate, see manualism and oralism.
When talking with a deaf person who uses speechreading, exaggerated mouthing of words is not considered to be helpful and may in fact obscure useful clues. However, it is possible to learn to emphasize useful clues; this is known as "lip speaking".
Speechreading may be combined with cued speech—movements of the hands that visually represent otherwise invisible details of pronunciation. One of the arguments in favor of the use of cued speech is that it helps develop lip-reading skills that may be useful even when cues are absent, i.e., when communicating with non-deaf, non-hard of hearing people.
See also 
- Audio-visual speech recognition
- Forensic speechreading
- Motor theory of speech perception
- Read My Lips (disambiguation)
- Reading (process)
- Silent speech interface
- Visual capture
- Kisor, Henry (2010), What's That Pig Outdoors?: A Memoir of Deafness, University of Illinois Press
- Clegg, Dorothy (1953), The Listening Eye: A Simple Introduction to the Art of Lip-reading, Methuen & Company
- CSAIL: Articulatory Feature Based Visual Speech Recognition - To develop a visual speech recognition system that models visual speech in terms of the underlying articulatory processes.
- an MRI video of a person speaking, showing tongue movements not visible to a lip reader