Keywen category structure.
The following two basic category systems are currently most popular:
- Hierarchical, as in directories (easier for understanding, planning and processing);
- Multi-hierarchical, as in Wiki-encyclopedias (more natural, flexible and easy to maintain).

Keywen category structure is the product of these two systems:
it has advantages of both and opens greater possibilities than either. 

Keywen category structure is more precise, logically correct, flexible and dynamic.
It is convenient for effective navigation and fast understanding, helps to see the BIG PICTURE,
helps to divide knowledge into parts and select the most important parts,
helps to create effective plans for learning and knowledge processing.