Both speaking and writing depend upon the underlying structures of language.
In order to distinguish pictures from pictorial signs, it is necessary to notice that language has two primary levels of structure, which the French linguist André Martinet referred to as the “double articulation” of language: the grammar as a system for mapping—establishing a system of relations between—sound and meaning.
These levels of structure admit of several subdivisions, any one of which may be captured in a writing system.
Phonemes may be further analyzed in terms of a set of underlying distinctive features, features specifying the ways the sound is physically produced by passing breath through the throat and positioning the tongue and lips.
Phonemes may be thought of as roughly equivalent to the sound segments known as consonants and vowels, and combinations of these segments make up syllables.
Such signs therefore express meanings, not thoughts, and they do so by representing meaning structures larger than can be expressed by a single word.