(sight), but may also exploit tactile features (tactile sign languages). Spoken language is by and large linear; only one sound can be made or received at a time. Sign language, on the other hand, is visual and, hence, can use a simultaneous expression, although this is limited articulatorily and linguistically. Visual perception allows processing of simultaneous information. One way in which many sign languages take advantage of the spatial nature of the language is through the use of classifiers. Classifiers allow a signer to spatially show a referent's type, size, shape, movement, or extent. The large focus on the possibility of simultaneity in sign languages in contrast to spoken languages is sometimes exaggerated, though. The use of two manual articulators is subject to motor constraints, resulting in a large extent of symmetry or signing with one articulator only. Further, sign languages, just like spoken languages, depend on linear sequencing of signs to form sentences; the greater use of simultaneity is mostly seen in the morphology (internal structure of individual signs). Non-manual elements Sign languages convey much of their prosody through non-manual elements. Postures or movements of the body, head, eyebrows, eyes, cheeks, and mouth are used in various combinations to show several categories of information, including lexical distinction, grammatical structure, adjectival or adverbial content, and discourse functions. At the lexical level, signs can be lexically specified for non-manual elements in addition to the manual articulation. For instance, facial expressions may accompany verbs of emotion, as in the sign for angry in Czech Sign Language. Non-manual elements may also be lexically contrastive. For example, in ASL (American Sign Language), facial components distinguish some signs from other signs. An example is the sign translated as not yet, which requires that the tongue touch the lower lip and that the head rotate from side to side, in addition to the manual part of the sign. Without these features the sign would be interpreted as late. Mouthings, which are (parts of) spoken words accompanying lexical signs, can also be contrastive, as in the manually identical signs for doctor and battery in Sign Language of the Netherlands. While the content of a signed sentence is produced manually, many grammatical functions are produced non-manually (i.e., with the face and the torso). Such functions include questions, negation, relative clauses and topicalization. ASL and BSL use similar non-manual marking for yes/no questions, for example. They are shown through