A written language is the representation of a spoken or gestural language by means of a writing system. Written language is an invention in that it must be taught to children, who will pick up spoken language (oral or sign) by exposure even if they are not specifically taught.
A written language exists only as a complement to a specific spoken language, and no natural language is purely written.
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Written languages change more slowly than corresponding spoken languages. When at least one register of a language is strongly divergent from spoken language, the resulting situation is called diglossia. However, that is still often considered as one language, between literary language and other registers, especially if the writing system reflects its pronunciation.
Native readers and writers of English are often unaware that the complexities of English spelling make written English a somewhat artificial construct. The traditional spelling of English, at least for inherited words, preserves a late Middle English phonology that is no one's speech dialect. The artificial preservation of that form of the language, in writing, might make much of what is now written intelligible to Geoffrey Chaucer (1343–1400) even if the mediaeval writer's speech could no longer be understood.