duality （of structure）
Duality (of structure) is one design feature of language which means that language is a system consisting of two sets of structures,or two levels.At the lower or the basic level there is a structure of sounds, which are meaningless .But the sounds of language can be grouped and regrouped into a large number of units of meaning such as morphemes and words,which are found at the higher level of the system.For example,the grouping of the three sounds/k/,/a:/,and /p/ can mean either a kind of fish(carp),or a public place for rest and amusement (park).
