Core and Peripheral Grammar
In UG core and peripheral grammar can be defined as a set of rules that a child learns as a part of its language acquisition. According to UG there are certain universal principles and parameters that form the framework of our mind. With the help of this framework a child develops its language. The universal rules that a child discovers form the core grammar of its language. And the principles which are unique are known as peripheral. The whole complex apparatus is concerned with the crucial central area of syntax defined as core grammar .But much of language is peripheral, idiosyncratic and linked to UG in a looser way.
Chomsky developed the idea that each sentence in a language has two levels of representation — a deep structure and a surface structure. The deep structure represented the core semantic relations of a sentence, and was mapped on to the surface structure via transformations. Chomsky believed that there would be considerable similarities between languages' deep structures, and that these structures would reveal properties, common to all languages, which were concealed by their surface structures. According to him the rules that the child discovers in a language with help of UG form the core grammar of his language.Not all rules are core rules. Every language also contains elements that are not constrained by UG. These comprise the periphery. Usually the peripheral rules are those that are derived from the history of language, that have been borrowed from other languages, or that have arisen accidentally. Thus,the child’s knowledge of his mother tongue is made up of rules determined by UG(the core) and those that have to be learnt without the help of UG(the periphery).