The design features refer to the defining features of human languages that tell differences between human language and any system of animal communication. \nArbitrariness is a core feature of language, which means that there is no logical connection between meanings and sounds. A good example is the fact that different sounds are used to refer to the same object in different languages. But we should aware that language is not entirely arbitrary. Certain words are motivated. The best examples are onomatopoeic words, such as crash and bang. Besides, some compound words are also not entirely arbitrary. While photo and copy are both arbitrary, the compound word photocopy is not arbitrary. Language is creative and productive in that it makes possible construction and representation of new signals by its users. This is why they can produce and understand an infinitely large number of sentences, including sentences they have never heard before. Language is a system, which consists of two sets of structures or two levels. At the lower or the basic level there is a structure of sounds, which are meaningless by themselves. But the sounds of language can be grouped or regrouped into a relatively large number of units of meaning, which are found at the higher level of the system. 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 far away places. In other words, language can be used to refer to context removed from immediate situation of the speaker. This property provides the speaker with an opportunity to talk a wide ranger of things, free from barriers caused by separation in time and space. Language is passed on from one generation to the next through teaching and learning, rather than by instinct. Interchangeability means that human being can be both a producer and a receiver of messages. \nArbitrariness 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.