The narrow sense chain-making and chain-coupling theorems of element grid in 2-D problems

The purpose of this paper is to answer a basic problem of finite element method that is how do the elements which have determined shapes form the element grid which has determined structure. In order to study this problem, a method called chain-making and chain-coupling is established. Through this method the qualified relationships between the geometric structures of the element grid and the geometric structures of its elements are founded. This problem is studied both in 2-D and 3-D cases and here is the first part of 2-D case conclusions.