How much of your codebase is only understood by one person?

What an optimistic question!

Try: "How much of your codebase isn't understood by any people at all?" :)

To me it all depends on the size of the codebase / the complexity of the project. Not the complexity of the code.

The bus factor is not a question of so complex code that only N people understand it.

To add to the discussion, do you people think that programming has enough "meta" tools ? (I don't know how to put it.) I mean, we have stuff like javadoc by example, which can help documenting (implies generally understanding) a codebase. But could we think of a really good way/tool to help understand a codebase, be its code simple or complex ?

When we accept complexity while striving for simplicity, we end up with actual features.