Linguists have found that sign languages and spoken languages share many features. Like spoken languages, which use units of sounds to produce words, sign languages use units of form. These units are composed of four basic hand forms: hand shape, such as an open hand or closed fist hand location, such as on the middle of the forehead or in front of the chest hand movement, such as upward or downward and hand orientation, such as the palm facing up or out. In spoken languages units of sound combine to make meaning. Separately, b, e, and t have no meaning. However, together they form. the word bet. Sign languages contain units of form. that by themselves hold no meaning, but when combined create a word. Spoken languages and sign languages differ in the way these units combine to make words, however. In spoken languages units of sound and meaning are combined sequentially. In sign languages, units of form. and meaning are typically combined simultaneously. In American Sign Language (ASL) signs follow a certain order, just as words do in spoken English. However, in ASL one sign can express meaning that would necessitate the use of several words in speech. For example, the words in the statement 'I stared at it for a long time' each contain a unit of meaning. In ASL, this same sentence would be expressed as a single sign. The signer forms 'look at' by making a V under the eyes with the first and middle fingers of the right hand. The hand moves out toward the object being looked at, repeatedly tracing an oval to indicate 'over a long time'. To express the adverb 'intently' the signer squints the eyes and purses the lips. (To purse the lips is like saying mmmm: pull back and tighten the lips with the lips closed.) Although the English words used to describe the ASL signs are written out in order, in sign language a person forms the signs 'look at', 'long time', and 'intently' at the same time. ASL has a rich system for modifying the meaning of signs. Verbs such as 'look at' can be changed to indicate that the activity takes place without interruption, repeatedly, or over a long time. The adjective 'sick', for example, is formed by placing the right middle finger on the forehead and the left middle finger on the stomach. By forming the sign 'sick' and repeatedly moving the left hand in a circle, the signer can indicate that someone is characteristically or always sick. Facial grammar, such as raised eyebrows, also can modify meaning. For example, a signer can make the statement 'lie is smart' by forming the ASL sign for 'smart' --placing the middle finger at the forehead -- and then quickly pointing it outward as if toward another person to indicate 'he'. To pose the question 'Is he smart?' the signer accompanies this sign with raised eyebrows and a slightly tilted head. People who sign sometimes use finger spelling to represent letters of the alphabet. In some sign languages, including ASL, finger spelling serves as a way to borrow words from spoken language. A deaf person might, for example, choose to fingerspell 'd-o-g' for 'dog' instead of using a sign. Several types of finger spelling systems exist. Linguists still have much to learn about the world's sign languages. What has become clear is that hundreds, if not thousands, of sign languages exist around the world. According to the passage, which of the following statements is TRUE?