The two-level directory system even on a single-user PC, is inconvenient.
Since it is common for the users, want to group their files together in logical ways.
Therefore, some way is needed is a general hierarchy.
Here, hierarchy means a tree of directories.
With hierarchical approach, each user on the computer system can have as many directories as are needed, so that files can be grouped together in natural ways.
The figure given below shows this hierarchical directory system:
As you can see from the above figure, the directories X, Y, and Z, contained in the root directory where each belong to a different user, two of them have created the subdirectories for the project they are working on.