Conditional tag for sub-pages

Hey everyone… So, I’m not sure if I’ll get all of the WP terminology correct, but the gist of what I want to learn how to do do is the following.

How can I set things up so that when I’m on any *sub-page* of *page* X, there is a list of all other *sub-pages* of *page* X listed in the side-bar of the *sub-page*. Did that make sense? I hope so.

Thanks…