[texhax] \overline and \phantom

[Paul Stanley]

[...]
 > something like that, basically \phantom{...} takes up the same space as
 > ... would
 >
 > there are two variants as well, \vphantom who give the same height and
 > depth, but no width, and \hphantom, that provide width but no depth or
 > height.
 >
 > Note that none of the phantoms print their argument
 >
 >
 > At the other end there is \smash, which is like \hphantom, but it also
 > prints its argument

Could you provide examples of where `\vphantom' and `\hphantom' could be used?
I can only picture two dimensions on the page, height and width, 
where does the depth fit into this?
Finally, where would smash be used?

Many thanks,
Paul