Subject Strange behaviour on Linux

When I have the following directory structure on a Linux server:


and I want to create a database /a/b/c/test.fdb with gbak, the access rights of directory "c" must be rwx (readable, writeable, executable). Also the access rights to directory "a" and "b" must be r_x (readable, executable). The same with use of isql.
My question is why?
There might be some (unknown to me) reason that directory "c" must be executable, but why also directory "a" and "b"?

Can somebody explain this to me? I don't understand it.

With kind regards,

Roy Damman

This applies to 2.5.x but I think also to 3.x.