The Hierarchical Data Model:

History: This model was developed by default during the
'60s as the concept of computerized databases was evolving.

A popular example of this structure was IBM's Information 
Mgt System (IMS) which is still in use with some applications.
The relationships within a hierarchical model are generally
IS_PART_OF or IS_A relationships.

The Hierarchical model uses the "Inverted Tree" Analogy with
(a root, branches, twigs, leaves)   Another analogy is the 
parent/child analogy, which leads to the following rules:

1. Each child can have only one parent.
2. Every child inherits the attributes of its parent.