Page "American Sign Language" Paragraph 33

Since in oral languages the elements of sound are for the most part produced linearly in time ( that is, in a word like cat the a sound comes after the c sound, and the t sound comes after that ), they can generally be easily written in a linear ( one-dimensional ) writing system such as an alphabet.

There are only a few aspects which are produced simultaneously, such as tone in some languages, or vowel and consonant harmony, and which are therefore not straightforward to write with an alphabet.

In sign languages, however, several channels operate simultaneously — hand shape, often with the two hands operating independently, hand location, hand motion, facial expression, mouthing — making an alphabetic script more complicated than just stringing letters together in the order sounds are produced.