articulation takes place. However, although a language may contrast pre-velar and post-velar sounds, it does not also contrast them with palatal and uvular sounds (of the same type of consonant) so contrasts are limited to the number above, if not always their exact location. Table of gestures and passive articulators and resulting places of articulation The following table shows the possible combinations of active and passive articulators. The possible locations for sibilants as well as non-sibilants to occur are indicated in dashed red. For sibilants, there are additional complications involving tongue shape; see the article on sibilants for a chart of possible articulations. A precise vocabulary of compounding the two places of articulation is sometimes seen. However, it is usually reduced to the passive articulation, which is generally sufficient. Thus dorsal–palatal, dorsal–velar, and dorsal–uvular are usually just called "palatal", "velar", and "uvular". If there is ambiguity, additional terms have been invented, so subapical–palatal is more commonly called "retroflex". NOTE: Additional shades of passive articulation are sometimes specified using pre- or post-, for example prepalatal (near the border between the postalveolar region and the hard palate; prevelar (at the back of the hard palate, also post-palatal or even medio-palatal for the middle of the hard palate); or postvelar (near the border of the soft palate and the uvula). They can be useful in the precise description of sounds that are articulated somewhat farther forward or back than a prototypical consonant; for this purpose, the "fronted" and "retracted" IPA diacritics can be used. However, no additional shade is needed to phonemically distinguish two consonants in a single language. Homorganic consonants Consonants that have the same place of articulation, such as the alveolar sounds in English, are said to be homorganic. Similarly, labial and velar are homorganic. A homorganic nasal rule, an instance of assimilation, operates in many languages, where a nasal consonant must be homorganic with a following stop. We see this with English intolerable but implausible; another example is found in Yoruba, where the present tense of ba "hide" is mba "is hiding", while the present of sun "sleep" is nsun "is sleeping". Central and lateral articulation The tongue contacts the mouth with a surface that has two dimensions: length and width. So far, only points of articulation along its length have been considered. However, articulation varies along its width as well. When the airstream is directed down the center of