A written language is a language that has a writing system, or more generally the written form of any language that has such written components.
Written language is an invention; whereas, spoken language has evolved along with homo sapiens. Children will instinctively learn or create spoken (or gestural) languages. However, written language must be taught.
Written language always appears as a complement to a specific natural language (English, French, American Sign Language, etc.) and no purely written languages exist.
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