In general, hierarchies can be either balanced or ragged.
In a balanced hierarchy, the parent of every member comes from the level immediately above the member. A family tree is an example of a balanced hierarchy. Most hierarchies on the time dimension are also balanced.
Ragged hierarchy is the more common type. In a ragged hierarchy, the parent of a member can come from any level above the level of the member, not just from the level immediately above.
Many hierarchies in a multidimensional schema are not balanced. For example, the hierarchy in a Geography dimension is usually not balanced — at least, it is not balanced conceptually. This is because some countries are divided into states, and states have counties and cities. However, other countries have regions and cities, but no states.
OLE DB for OLAP supports both balanced and ragged hierarchies. The MEMBERS schema rowset has not only a parent name, but also the level number of the parent. Similarly, the restrictions on this rowset enable you to browse not only by moving up and down the levels, but also by moving up and down the inheritance tree.