Early mainframe DBMS packages used the hierarchical structure , in which the relationships
between records form a hierarchy or treelike structure. In the traditional hierarchical
model, all records are dependent and arranged in multilevel structures, consisting of
one root record and any number of subordinate levels. Thus, all of the relationships
among records are one-to-many because each data element is related to only one element
above it. The data element or record at the highest level of the hierarchy (the department
data element in this illustration) is called the root element. Any data element can
be accessed by moving progressively downward from a root and along the branches of
the tree until the desired record (e.g., the employee data element) is located.