Arbitrariness is a core feature of language, which means that there is no logical connection between meanings and sounds. Duality, which means the property of having two levels of structures, such that units of the primary level are composed of elements of the secondary level and each of the two levels has its own principles of organization. Creativity means language is resourceful because of its duality and its recursiveness. Because of duality the speaker is able to combine the basic linguistic unites to form an infinite set of sentences, most of which are never heard before. Displacement means that language can be used to refer to things which are present or not present, real or imagined matters, in the past, present, or future, or in far-away places. So all these features make language unique to human beings.