How to represent a tree-structure in a relational database

I am just thinking about the data model for a little content management system that I am
currently planning. Individual articles are sorted under different categories which branch
into subcategories, sub-subcategories etc. up to a depth of about 6 or 7 levels. The
structure should be extensible, i.e. it must be possible to add levels. What I am thinking
now is that you would keep the index in a separate index table (linked with the primary
key in the articles table), which would have 6 or 7 fields initially, and that you'd add
columns with the alter table command, if need be, to make the structure deeper. Is this
the recommended way to go about it? It feels pretty 'right' to me now but since the
problem should be fairly common, there must be other people who have thought and written
about it and there might even be a recognized 'optimal' solution to the problem.

Comments?

- Frank