Language is structured at multiple levels, beginning in the case of spoken language with patterns in the acoustic signal that can be mapped to phones (the distinguishable successive sounds of which languages are built up). Groups of phones that are equivalent for a given language (not affecting the words recognized by a hearer, if interchanged) are the phonemes of the language. The phonemes in turn are the constituents of morphemes (minimal meaningful word segments), and these provide the constituents of words. (In written language one speaks instead of characters, graphemes, syllables, and words.) Words are grouped into phrases, such as noun phrases, verb phrases, adjective phrases and prepositional phrases, which are the structural components of sentences, expressing complete thoughts. At still higher levels we have various types of discourse structure, though this is generally looser than lower-level structure.