Easy floating <div> question

I just can't remember for the life of me

[mcvoid]

Hello, all. Just a simple question:
How do I get a relatively-positioned floating <div> to fill to the entire height of a parent container, so that the main content doesn't flow under the bottom of the floating part and stays where it's put?
I don't want to move the position of the main content- everything else is relative and it will screw up on a small viewport.

[mcvoid]

No reply? That's too bad. Usually you guys are quick with an answer.

[londrum]

i'm not sure you can do that so simply.

if it is inside another container, then you could apply a padding to the lefthand-side of the container so that the main content cannot flow in there, and then position your div absolutely, so it appears inside that blank space.