Topological relations have been recognized to be very useful for spatial query

analysis and reasoning.This paper concentrates on the topological relations between two lines in IR2.The line of thought employed in this study is that the topological relation between two lines can be described by a combination of finite number of basic（or elementary） relations.Based on this idea

a hierarchical approach is proposed for the description and determination of basic relations between two lines

where the topological invariants

including dimension

the number of separates

type of components

and sequence of components in the intersection of two lines

are defined and utilized.Seventeen（17） basic relations are identified and eleven（11） of them form the basis for combinational description of a complex relation

which can be determined by a compound relation model.