Still-larger units make up such discourse structures as propositions and less well-defined units of meaning such as prayers, stories, and poems.
The basic linguistic unit of the sound system is called a phoneme; it is a minimal, contrastive sound unit that distinguishes one utterance from another.
Consequently, writing cannot ordinarily be read by someone not familiar with the linguistic structure underlying the oral form of the language.
Writing systems can serve to represent any of these levels of sound or any of the levels of meaning, and, indeed, examples of all of these levels of structure have been exploited by some writing system or other.
Writing systems consequently fall into two large general classes: those that are based on some aspect of meaning structure, such as a word or a morpheme, and those that are based on some aspect of the sound system, such as the syllable or the phoneme.
These alternative relations may be depicted as follows: It is the fact that writing is an expression of language rather than simply a way of transcribing speech that gives to writing, and hence to written language and to literacy, its special properties.
As long as writing was seen merely as transcription, as it was by such pioneering linguists as Ferdinand de Saussure and Leonard Bloomfield earlier in the 20th century, its conceptual significance was seriously underestimated.