Viseme

Thus words such as pet, bell, and men are difficult for lip-readers to distinguish, as all look like alike.

However, there may be differences in timing and duration during natural speech in terms of the visual "signature" of a given gesture that cannot be captured by simply concatenating (stilled) images of each of the mouth patterns in sequence[2].

For example, in spoken English /l/ and /r/ can often sound quite similar (especially in clusters, such as 'grass' vs. 'glass'), yet the visual information can disambiguate.

Some linguists have argued that speech is best understood as bimodal (aural and visual), and comprehension can be compromised if one of these two domains is absent (McGurk and MacDonald 1976).

Visemes can often be humorous, as in the phrase "elephant juice", which when lip-read appears identical to "I love you".