This document discusses key features of human language that distinguish it from animal communication systems. It covers two modes of language: spoken and written. It then examines four unique properties of human language: arbitrariness, duality, creativity, and displacement. Arbitrariness refers to the arbitrary relationship between words and their meanings. Duality refers to language having two layers - sounds that combine to form meaningful units. Creativity comes from language's recursiveness allowing new meanings. Displacement allows referring to things not present in space or time. The document asks cases about a president wondering about many local languages versus one national language, and why English is used internationally. It also defines lingua franca, vernacular, pidgin,